Add hints to the variable bindings exercises

variables/variables4.rs

@@ -1,6 +1,44 @@
-// Make me compile!
+// Make me compile! Scroll down for hints :)
 
 fn main() {
     let x: i32;
     println!("Number {}", x);
 }
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+// Oops! In this exercise, we have a variable binding that we've created on
+// line 4, and we're trying to use it on line 5, but we haven't given it a
+// value. We can't print out something that isn't there; try giving x a value!
+// This is an error that can cause bugs that's very easy to make in any
+// programming language -- thankfully the Rust compiler has caught this for us!