What am I forgetting about 4e D&D?

[Kurald Galain]

I did have to put in a restriction early on that Tier 1 (1-4) and Tier 2 (5-10) required different groups, given the huge jump in power in 5e at level 5 ... 4e couldn't handle a party spread of more than about 3 total levels.

So that's pretty much the same, then. And actually, the same as in 3E/PF as well. You can have a L6 and L9 in the same group just fine, but don't try L2 and L9, obviously.