update readme progress

Author: destinationunknown

README.md

@@ -12,6 +12,7 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 1](https://adventofcode.com/2024/day/1) | ⭐ | ⭐ |
 | [Day 2](https://adventofcode.com/2024/day/2) | ⭐ | ⭐ |
 | [Day 3](https://adventofcode.com/2024/day/3) | ⭐ | ⭐ |
+| [Day 4](https://adventofcode.com/2024/day/4) | ⭐ |   |
 <!--- advent_readme_stars table --->
 
 <!--- benchmarking table --->