Thanks a lot for playing the game!
We wanted to introduce all the different enemies and ships gradually, but we ran short of time... the third (red) one was rushed and the fourth (yellow) was slapped in at the very end of the jam, leaving the level unfinished. Also, we wanted to introduce more enemy types, and the shooting flowers were also rushed.
The idea was to introduce each new ship before a dedicated enemy wave. Unlocked ships would then appear even in non-dedicated waves.
Glad you had fun with this proof of conceot of ours! :D