Update practicalSession-4.md

practicalSession-4.md

@@ -27,6 +27,9 @@ Then, install the tutorials:
 `curl -L https://raw.githubusercontent.com/rust-lang/rustlings/main/install.sh | bash`
 
 
+## Running Rustlings, the Rust tutorials
+
+
 `cd rustlings`
 
 
@@ -36,3 +39,11 @@ Finally run to launch the exercices:
 
 
 Follow the given instructions while editing `rs` files with a text editor.
+
+
+Focus on code quality:
+- `variables4.rs`, what is the difference between mutability and immutability? Which relation with code safety? 
+
+
+
+