Update Exercises Directory Names to Reflect Order

exercises/02_functions/functions4.rs

@@ -0,0 +1,28 @@
+// functions4.rs
+//
+// This store is having a sale where if the price is an even number, you get 10
+// Rustbucks off, but if it's an odd number, it's 3 Rustbucks off. (Don't worry
+// about the function bodies themselves, we're only interested in the signatures
+// for now. If anything, this is a good way to peek ahead to future exercises!)
+//
+// Execute `rustlings hint functions4` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+fn main() {
+    let original_price = 51;
+    println!("Your sale price is {}", sale_price(original_price));
+}
+
+fn sale_price(price: i32) -> {
+    if is_even(price) {
+        price - 10
+    } else {
+        price - 3
+    }
+}
+
+fn is_even(num: i32) -> bool {
+    num % 2 == 0
+}