I think level 4 is a really good level cap for EA currently. Its super easy to do all of the fights over-leveled Above Ground without actually trying, but once you get to
the Hag (When she's not bugged...) or the Underdark, things get way more difficult. The Gith patrol is actually pretty challenging if you're not expecting them to attack you and didn't preemptively take high ground.
It's really just a good balance for testing encounters currently, and a spell like fireball would oversimplify a lot of them.
Should they take a different route than they did with DOS:2, and continue adding content, then yeah anything past where we are would hopefully start needing more tools than we have access to right now.
Hard disagree.
It's pretty clear that you can get to level 5 even doing just half of what Act 1 offers, I am pretty much level 5 XP-wise just by saving Druid Grove plus Underdark, without even touching the rest like Swamp, Gith, Inn, Gnolls and so on.
If the XP gain remains as it is you will see people leaving Act 1 as level 6 or near that out of their intended 10 levels total for a game, so that's a valid feedback there too that if getting to level 5 will invalidate everything in Act 1, then maybe it needs to be adjusted in some way, as in more level 5 encounters further in on less XP tossed around.
Other than that - I think it's ok for Larian to toss players a bone there fun-wise. Level 5 is a big power boost, you get to see new spells, new abilities and so on. This EA will last a LOOOOOONG time, I bet easy 2 years, so I do hope 2-3 months down the road they will unlock level 5 and maybe even do us a solid and make some sort of Arena or some such so we can slam some random NPC encounters while we wait for actual release for 2 years.