Currently thought I feel reaching lvl 4 went too fast. 1-3 is okey that it's relatively fast, because level 1 and 2 are considered the tutorial levels (many campaigns start at 3 because that's when the fun starts).
I agree, you get to level 4 too quickly and can reach the experience for level 5 long before you are ready to take the boat. I think you should only just barely make it to level 5 in the area we have available to us. Actually I think 3 is a little too quick also. But I'm sure Larian will be doing plenty of level balancing.