Fix part4 readme

intro/part4/README.md

@@ -1,20 +1,13 @@
-# Part 3
+# Part 4
 
-This is just like last time, except first we'll install the `elm/browser` package so we can create an interactive app in the browser, instead of static HTML.
+Build everything the same way as last time:
-
-To install the package, `cd` into the `part3/` directory and run:
-
-```shell
-elm install elm/browser
-```
-
-Then build everything the same way as last time:
 
 ```shell
 elm make src/Main.elm --output elm.js
 ```
 
-Finally, open `index.html` in your browser.
+No need to open `index.html` in your browser, since this time the exercise
+is only about compiling!
 
 ## Exercise