Tried the web build on desktop Chrome. It enabled with no fuss after the first load.
My read, one line: stage 4 isn't a balance wall yet, it's a legibility wall.
What I saw. The staging screen, where you choose your party, shows your roster, your six slots, and three hints. It shows nothing about the enemy you're about to face. So the party choice gets made blind.
Then in the fight the answer is there but hidden. wburd found half of it by accident: "after playing for a while I realised I could hover over the enemies and it showed they had a large magical resistance." Promoter found the other half: a Ranger that hits the back row, which is also a rule you only learn by trying it.
Both volunteers cleared stage 4 the same way: by discovering a rule the game was keeping. That's the pattern, and it isn't the encounter numbers. A blind party choice meets a hidden counter, and the player reads the loss as "I'm not strong enough" when it was really "I didn't know the rule."
Test I'd run: hand a fresh player stage 4 with no outside reading. If they can't clear it, it's legibility, not tuning.
The fix that follows: show the enemy's composition and traits on the staging screen, where the choice is made, instead of on hover, where the fight is already lost. The merge hint belongs in the same pass. The current hint sits in a corner next to two lines of flavor text, so it reads as atmosphere, not instruction.
Caveat, so you know which part is mine: I loaded the build and read the staging screen and this thread. I didn't clear stage 4 myself. The stage 4 specifics above are wburd's and Promoter's, quoted.