For the scale they're clearly wanting to portray, a party of 4 is way too small. When (I hope not if - they really need to) normalize the 5E implementation, this should be an obvious realization. In TT, 4 people, no matter their skill level are supremely limited before level 5. That's where the full featured-ness of D&D comes online. You don't even define your class kit until level 3.
You're literally just a rando before that point rules wise. So, you're NOT going to be fighting hags, storming goblin war camps, adventuring in the underdark, and so on. If you try, the rules say you die in most circumstances. These types of missions are absolutely level 5+, and I'd not feel comfortable doing the latter two until after level 7 especially the underdark. That is a scary, scary place.
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.
