Update README stars for year.

Author: obalunenko

README.md

@@ -35,6 +35,7 @@ This repository contains solutions for puzzles and cli tool to run solutions to
 | [Day 1](https://adventofcode.com/2021/day/1) | ⭐ | ⭐ |
 | [Day 2](https://adventofcode.com/2021/day/2) | ⭐ | ⭐ |
 | [Day 3](https://adventofcode.com/2021/day/3) | ⭐ | ⭐ |
+| [Day 4](https://adventofcode.com/2021/day/4) | ⭐ | ⭐ |
 <!--- advent_readme_stars table [2021] --->
 
 <!--- advent_readme_stars table [2020] --->