There's a general (not universal, some people love it!) consensus that Mega Man IV is where the original Mega Man series started to decline a bit, and it's usually attributed to either 1) the music got worse (true IMO, although it's still pretty good) or 2) adding the charge shot broke the game. 2 comes with three sub-reasons: A, you're incentivized to hold the fire button the entire time you play, which doesn't feel good; B, it makes an annoying high-pitched whine that cuts out half of the music in a series famous for its music; and C, most subtly, they had to turn all the enemies into bullet sponges to justify the existence of the charge shot, so the brisk pace of the game bogs down, the whole thing feels like a slog. It's a bummer, because MM4 is still near-peak Capcom and one of the best NES games, with nice colorful graphics, still pretty good music, and overall a high level of polish. 

Anyway, I thought it would be interesting to see if the charge-killed-Mega-Man theory (which I tend to believe myself) was actually right, so here's a hack that removes the charge shot and then makes a number of small balance adjustments to try to restore the pace of MM2/3. The main one is nerfing a number of enemies by a few HP, which brings them in line with equivalent enemies in MM3. You can read the rest below.

I'm filing this under "improvement," but if there were an "improvement?" category I'd put it there. It feels better to me, but I know some people genuinely like the charge, so your mileage may vary. See what you think!

FULL CHANGE LIST:

Remove charge shot
Drop Gachappon HP to 8 (was 12)
Drop Jumbig HP to 10 (was 14)
Drop Rackaser (umbrella guy) HP to 3 (was 4)
Drop Totem Polen HP to 6 (was 8)
Drop Wall Blaster HP to 4 (was 5)
Drop Escaroo (snail miniboss) HP to 10 (was 16)
Drop Whopper (spring miniboss) HP to 5 (was 9)
Drop Moby (whale miniboss) HP to 12 (was 14)
Drop Kabatoncue (hippo miniboss) HP to 10 (was 16)
Kabatoncue column segments take 1 hit but regrow faster
Cut repeats of Kabatoncue & Whopper in Ring Man (honestly I probably should've cut Whopper entirely, but he's in there)
Cut Robot Master invincibility frames from 48 to 28 to match Mega Man 3
Cut Robot Master contact damage from 8 to 6 (for everyone except Pharaoh Man, who already does 4, and Toad Man, who I felt sorry for)
Raise Skeleton Joe HP to 5 (was 3) but remove his collapse so you can actually kill him without the charge
Lower Wily Machine phase 2 by 8 pixels so you can hit him without the charge

Thanks to Matrixz and MartsINY for their comprehensive documentation of Mega Man 4's code, and to Raidenthequick for their complete MM4 disassembly.