This commit introduces Level 4 of the Alien Rush tower defense game including:
- New level map data (level4.json and level4.tmx) with terrain configuration
- Updated tileset references to support additional terrain types
- Complete wave configuration for Level 4 with 10 waves containing various enemy combinations
- README documentation updates reflecting the new level implementation
The changes enable players to progress through a new challenging level with updated enemy spawn patterns and increasing difficulty curves.
