Add chapter four (#391)

* Use js-slang v0.1.11

* Add chapter 4 to playground

* Bump js-slang

* Bump js-slang hopefully for the last time

Author: remo5000

package.json

@@ -37,7 +37,7 @@
     "draft-js": "^0.10.5",
     "flexboxgrid": "^6.3.1",
     "flexboxgrid-helpers": "^1.1.3",
-    "js-slang": "0.1.10",
+    "js-slang": "0.1.13",
     "lodash": "^4.17.10",
     "lz-string": "^1.4.4",
     "moment": "^2.22.2",

src/reducers/states.ts

@@ -161,7 +161,7 @@ export enum Role {
  * Defines what chapters are available for usage.
  * For external libraries, see externalLibraries.ts
  */
-export const sourceChapters = [1, 2, 3]
+export const sourceChapters = [1, 2, 3, 4]
 const latestSourceChapter = sourceChapters.slice(-1)[0]
 
 const currentEnvironment = (): ApplicationEnvironment => {