Dealing with a goblin encampment and fighting a hag is very doable before level 5. A green hag is CR3, goblins are Cr1/4, a goblin chief is CR 1.That's well within what a level 2-3 party can accomplish with some smart planning.
But to contribute something OT:
4 does feel pretty limited, Personally I like having a party of 5, but i think 6 is reasonable. That lets you explore half the classes in one play through. And gives you enough room to cover all your bases comfortably. That is of course assuming that we will get more (and hopefully a bit less over the top) companions.
A green hag may be CR3 but is definitely on the upper end of hard. One attack can pretty reliably one shot someone at level 2 which is when you can meet it in BG3. Goblins are CR 1/3 but this ignores placement and numbers. Those encounters are much higher than that. Mind you that CR is 1 creature vs 4. We often face bands of 8 or more. Even the random potshot with a short bow doing 8d6+16 will drop most characters on their own.
Add in the bugbears, gnolls with multiattack, and so on, and it definitely falls on the upper end regardless of the situation you come across per book rules. CR only makes sense as long as the party comp is constant. The moment you lose someone that's a drop of 25%. The relative CR goes up then and you can quickly crest into a death spiral. CR in general carries a lot of problems and this only highlights them. It isn't the defeater argument you think it is.
