If this wasn't reported already which I can't find, the happy ghast hitbox is slightly larger than 4x4x4, at least horizontally
To reproduce:
Make a hole or at least area that is 4x4, and make it so a 4x4 object would fit exactly without escaping. The images show a few pillars but those should work the same as a hole.
Place two blocks on the corner of the rim, so that you can line up the ghast perfectly.
Try and look down and move forward to go downwards.
Expected result: the ghast slides in perfectly, and stays healthy.
Actual result: the ghast cannot slide in, no matter how hard you try. Forcing it into the space makes it suffocate.
More information: I had a ghast who was contained using pillars like those in the images, and it died of suffocation in the update the bug occurred (which wasn't at release of chase the skies). Even pushing the ghast into place will lead it to clip through blocks
Did you apply any addons to the world?