Maybe, but i dont' see why they cant balance the 4 man content around the 4 players. The game really needs more broader difficulty, its either easy 4 man content, easy 8 man content, harder 8 man content, easy 24 man content. A harder 4 man content would be a nice addition.
You can, but balance around 4 and balance around 5/6 is different, the same way balance around 8 and 10 and 12 is wildly different.

4 man balance cant have multiple miniboss mechanics that have to be tanked by a different person, or require significant burst healing checks that one healer alone couldnt handle, or a good number of things you can easily do with 5, because the lack of the 5th party slot is huge (similar to how WoW's 5 man hard modes in the first few expansions came nowhere near what EQ's single group trials could throw at you with 6 people, because it could assume things about your group that 5mans couldnt).

1 healer and 1 tank significantly limit the types of mechanics that can be thrown at you.

You're left with either the method of everything having more hp and more damage output, or you get into mechanics where the party gets split a lot. Or you'll end up with mechanics that try and push people off a stage in space.

One of my favotire things in raid design was figuring exactly what you could throw at a group of people and expect them to, by default, handle. 10 mans in WoW was one of the absolute best sweet spots, because you could count on 2.5 tanks, 2.5 healers and 6 dps at different stages of the fight.

14's 8 mans feel a lot worse in that regard.

Similarly, cutting down to 4 feels AWFUL for meaningful design.