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.
Bumping it up to 6 not only lets more people from your camp join (which there will be even more companions coming later) but also matches the scale of the world they're being thrown in. This also reduces the reliance on all the surface shenanigans and puts the focus on the player characters which is the core thesis of D&D - gather friends and kill monsters.
