Really depends on what model of IV (for armor), and what for a gun (kwk 37/L 24, 40/L 43, 40/L 48) 75mm, and what year this all came to pass.
As T.A. stated 35-40mm on sides, and 80mm (like the H) on front, and the kwk40/L48 gun (apron armor all around) in 1939. That would be something.
Perhaps less trained crews would've been lost, as were in the much lesser junk they started the war with.
Think of all the gas saved by not having to spend it on those early suicide machines (I-II-III's) that even Japanese tanks could whip.
That and dumping the, III's to combat tanks and the IV's to support infantry "doctrine". Have both AP & HE aboard to fulfill both missions.
The T-34 wouldn't have been so fearsome. The Russians seem to have figured out that one (tank) size, fits (most) all (situations).
Make Marders out of the rest of the captured/antique/allied chassis's, but make sure they have at least the Russian 76.2 (crash-boom gun), or the L/48.

Weights counter balance long guns. The M-10/M-36 had some heavy iron on the rear of it's turret to balance that x-naval gun & that 90, as did others.

Minor modifications, as T.A. stated do not require major overhauls/refits/engineering/castings...ie... Designs/blueprints/time. A glorified "field" modification, done at the factory.
Really a big nothing, once you train the Monkeys doing it.

just another opinion

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