add macros4 example

macros/macros4.rs

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+// Make me compile! Scroll down for hints :)
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+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    }
+    ($val:expr) => {
+        println!("Look at this other macro: {}", $val);
+    }
+}
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+fn main() {
+    my_macro!();
+    my_macro!(7777);
+}
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+// You only need to add a single character to make this compile.
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+// The way macros are written, it wants to see something between each "macro arm", so it can
+// separate them.