diff --git a/chapter04/package-lock.json b/chapter04/package-lock.json
index 5cebb3c..5dad169 100644
--- a/chapter04/package-lock.json
+++ b/chapter04/package-lock.json
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+            "integrity": "sha1-IQmtx5ZYh8/AXLvUQsrIv7s2CGM="
         },
         "object-copy": {
             "version": "0.1.0",
@@ -4864,6 +5059,11 @@
                 "find-up": "1.1.2"
             }
         },
+        "platform": {
+            "version": "1.3.5",
+            "resolved": "https://registry.npmjs.org/platform/-/platform-1.3.5.tgz",
+            "integrity": "sha512-TuvHS8AOIZNAlE77WUDiR4rySV/VMptyMfcfeoMgs4P8apaZM3JrnbzBiixKUv+XR6i+BXrQh8WAnjaSPFO65Q=="
+        },
         "pluralize": {
             "version": "7.0.0",
             "resolved": "https://registry.npmjs.org/pluralize/-/pluralize-7.0.0.tgz",
@@ -5135,10 +5335,15 @@
                 "set-immediate-shim": "1.0.1"
             }
         },
+        "referrer-policy": {
+            "version": "1.1.0",
+            "resolved": "https://registry.npmjs.org/referrer-policy/-/referrer-policy-1.1.0.tgz",
+            "integrity": "sha1-NXdOtzW/UPtsB46DM0tHI1AgfXk="
+        },
         "regenerate": {
-            "version": "1.3.3",
-            "resolved": "https://registry.npmjs.org/regenerate/-/regenerate-1.3.3.tgz",
-            "integrity": "sha512-jVpo1GadrDAK59t/0jRx5VxYWQEDkkEKi6+HjE3joFVLfDOh9Xrdh0dF1eSq+BI/SwvTQ44gSscJ8N5zYL61sg==",
+            "version": "1.4.0",
+            "resolved": "https://registry.npmjs.org/regenerate/-/regenerate-1.4.0.tgz",
+            "integrity": "sha512-1G6jJVDWrt0rK99kBjvEtziZNCICAuvIPkSiUFIQxVP06RCVpq3dmDo2oi6ABpYaDYaTRr67BEhL8r1wgEZZKg==",
             "dev": true
         },
         "regenerator-runtime": {
@@ -5180,7 +5385,7 @@
             "integrity": "sha1-SdA4g3uNz4v6W5pCE5k45uoq4kA=",
             "dev": true,
             "requires": {
-                "regenerate": "1.3.3",
+                "regenerate": "1.4.0",
                 "regjsgen": "0.2.0",
                 "regjsparser": "0.1.5"
             }
@@ -5350,8 +5555,7 @@
         "safe-buffer": {
             "version": "5.1.2",
             "resolved": "https://registry.npmjs.org/safe-buffer/-/safe-buffer-5.1.2.tgz",
-            "integrity": "sha512-Gd2UZBJDkXlY7GbJxfsE8/nvKkUEU1G38c1siN6QP6a9PT9MmHB8GnpscSmMJSoF8LOIrt8ud/wPtojys4G6+g==",
-            "dev": true
+            "integrity": "sha512-Gd2UZBJDkXlY7GbJxfsE8/nvKkUEU1G38c1siN6QP6a9PT9MmHB8GnpscSmMJSoF8LOIrt8ud/wPtojys4G6+g=="
         },
         "safe-regex": {
             "version": "1.1.0",
@@ -5523,7 +5727,7 @@
                 "extend-shallow": "2.0.1",
                 "map-cache": "0.2.2",
                 "source-map": "0.5.7",
-                "source-map-resolve": "0.5.1",
+                "source-map-resolve": "0.5.2",
                 "use": "3.1.0"
             },
             "dependencies": {
@@ -5634,9 +5838,9 @@
             "dev": true
         },
         "source-map-resolve": {
-            "version": "0.5.1",
-            "resolved": "https://registry.npmjs.org/source-map-resolve/-/source-map-resolve-0.5.1.tgz",
-            "integrity": "sha512-0KW2wvzfxm8NCTb30z0LMNyPqWCdDGE2viwzUaucqJdkTRXtZiSY3I+2A6nVAjmdOy0I4gU8DwnVVGsk9jvP2A==",
+            "version": "0.5.2",
+            "resolved": "https://registry.npmjs.org/source-map-resolve/-/source-map-resolve-0.5.2.tgz",
+            "integrity": "sha512-MjqsvNwyz1s0k81Goz/9vRBe9SZdB09Bdw+/zYyO+3CuPk6fouTaxscHkgtE8jKvf01kVfl8riHzERQ/kefaSA==",
             "dev": true,
             "requires": {
                 "atob": "2.1.1",
@@ -6221,9 +6425,9 @@
             }
         },
         "upath": {
-            "version": "1.0.5",
-            "resolved": "https://registry.npmjs.org/upath/-/upath-1.0.5.tgz",
-            "integrity": "sha512-qbKn90aDQ0YEwvXoLqj0oiuUYroLX2lVHZ+b+xwjozFasAOC4GneDq5+OaIG5Zj+jFmbz/uO+f7a9qxjktJQww==",
+            "version": "1.1.0",
+            "resolved": "https://registry.npmjs.org/upath/-/upath-1.1.0.tgz",
+            "integrity": "sha512-bzpH/oBhoS/QI/YtbkqCg6VEiPYjSZtrHQM6/QnJS6OL9pKUFLqb3aFh4Scvwm45+7iAgiMkLhSbaZxUqmrprw==",
             "dev": true
         },
         "update-notifier": {
@@ -6412,12 +6616,22 @@
                 "signal-exit": "3.0.2"
             }
         },
+        "x-xss-protection": {
+            "version": "1.1.0",
+            "resolved": "https://registry.npmjs.org/x-xss-protection/-/x-xss-protection-1.1.0.tgz",
+            "integrity": "sha512-rx3GzJlgEeZ08MIcDsU2vY2B1QEriUKJTSiNHHUIem6eg9pzVOr2TL3Y4Pd6TMAM5D5azGjcxqI62piITBDHVg=="
+        },
         "xdg-basedir": {
             "version": "3.0.0",
             "resolved": "https://registry.npmjs.org/xdg-basedir/-/xdg-basedir-3.0.0.tgz",
             "integrity": "sha1-SWsswQnsqNus/i3HK2A8F8WHCtQ=",
             "dev": true
         },
+        "xtend": {
+            "version": "4.0.1",
+            "resolved": "https://registry.npmjs.org/xtend/-/xtend-4.0.1.tgz",
+            "integrity": "sha1-pcbVMr5lbiPbgg77lDofBJmNY68="
+        },
         "y18n": {
             "version": "3.2.1",
             "resolved": "https://registry.npmjs.org/y18n/-/y18n-3.2.1.tgz",
diff --git a/chapter04/package.json b/chapter04/package.json
index 37fbe17..fc6e929 100644
--- a/chapter04/package.json
+++ b/chapter04/package.json
@@ -21,6 +21,7 @@
     "babel": {
         "presets": [
             "env",
+            "node",
             "flow"
         ]
     },
@@ -72,12 +73,15 @@
     },
     "dependencies": {
         "axios": "^0.18.0",
+        "cors": "^2.8.4",
         "express": "^4.16.3",
+        "helmet": "^3.12.1",
+        "jsonwebtoken": "^8.2.1",
         "mariasql": "^0.2.6"
     },
     "devDependencies": {
         "babel-eslint": "^8.2.2",
-        "babel-preset-env": "^1.6.1",
+        "babel-preset-env": "^1.7.0",
         "babel-preset-flow": "^6.23.0",
         "eslint": "^4.19.0",
         "eslint-config-recommended": "^2.0.0",
@@ -86,7 +90,7 @@
         "flow-coverage-report": "^0.5.0",
         "flow-remove-types": "^1.2.3",
         "flow-typed": "^2.4.0",
-        "nodemon": "^1.17.3",
+        "nodemon": "^1.17.4",
         "prettier": "^1.11.1"
     }
 }
diff --git a/chapter04/src/cors_server.js b/chapter04/src/cors_server.js
new file mode 100644
index 0000000..915c3d5
--- /dev/null
+++ b/chapter04/src/cors_server.js
@@ -0,0 +1,15 @@
+/* @flow */
+"use strict";
+
+const express = require("express");
+const cors = require("cors");
+
+const app = express();
+
+app.use(cors());
+
+app.get("/", (req, res) => res.send("Server alive, with CORS!"));
+
+app.listen(8080, () =>
+    console.log("CORS server ready at http://localhost:8080/!")
+);
diff --git a/chapter04/src/helmet_world.js b/chapter04/src/helmet_world.js
new file mode 100644
index 0000000..2d9ddcf
--- /dev/null
+++ b/chapter04/src/helmet_world.js
@@ -0,0 +1,16 @@
+/* @flow */
+"use strict";
+
+const express = require("express");
+const app = express();
+
+const helmet = require("helmet");
+app.use(helmet());
+
+app.get("/", (req, res) => res.send("Server alive, with Express!"));
+
+app.listen(8081, () =>
+    console.log(
+        "Mini server (with Express) ready at http://localhost:8080/!"
+    )
+);
diff --git a/chapter04/src/jwt_server.js b/chapter04/src/jwt_server.js
new file mode 100644
index 0000000..beea246
--- /dev/null
+++ b/chapter04/src/jwt_server.js
@@ -0,0 +1,51 @@
+/* @flow */
+"use strict";
+
+const express = require("express");
+const app = express();
+const jwt = require("jsonwebtoken");
+
+const SECRET_JWT_KEY = "modernJSbook";
+
+const bodyParser = require("body-parser");
+app.use(bodyParser.urlencoded({ extended: false }));
+
+app.get("/public", (req, res) => {
+    res.send("the /public endpoint needs no token!");
+});
+
+app.use((req, res, next) => {
+    // First check for the Authorization header
+    const authHeader = req.headers.authorization;
+    if (!authHeader.startsWith("Bearer ")) {
+        return res.status(401).send("No token specified");
+    }
+
+    // Now validate the token itself
+    const token = authHeader.split(" ")[1];
+
+    jwt.verify(token, SECRET_JWT_KEY, function(err, decoded) {
+        if (err) {
+            // Token bad formed, or expired, or other problem
+            return res.status(403).send("Token expired or not valid");
+        } else {
+            req.decoded = decoded;
+            // Everything OK, keep processing the request
+            next();
+        }
+    });
+});
+
+app.get("/private", (req, res) => {
+    res.send("the /private endpoint needs JWT, but it was provided: OK!");
+});
+
+// eslint-disable-next-line no-unused-vars
+app.use((err, req, res, next) => {
+    console.error("Error....", err.message);
+    res.status(500).send("INTERNAL SERVER ERROR");
+});
+
+app.listen(8080, () =>
+    console.log("Mini JWT server ready, at http://localhost:8080/!")
+);