The 4 different types of aliens are just the frames in the sprites animation . Every time you click the "next wave button" (spButtonRestart) in event 4 it adds 1 to the gvAlienType and this is used to set animation frame after the alien sprite (spAlien) is spawned from a random spSpawnPoint in event 2.
the sprite for spAlien consists of 4 frames each with a different alien graphic - animation speed set to 0.
The spawn number and frequency are also set in event 4 ( global variables - gvNumberOfAliens and gvDelayPerAlienSpawn)