This is pretty simple; all the Pyukuumukus I hatch are female. Four
so far, and only females. I would't be surprised if this is a bug,
considering that Pyukumuku was buggy a lot.
Pyukumuku have a 50/50 chance of hatching male or female;
apparently the latter is more prominent for you. Bishop has hatched more
than one male Pyukumuku.
Probably not. 4 isn't really a lot of eggs to hatch. Try 30 or more
at least I would say. As after all some people don't hatch a female
starter in over 100 eggs (my mum for example)