The SU-76M isn't a tank. It's a support gun, it's meant to support infantry by knocking out softening up defenses with barrages and knocking out bunkers, buildings and vehicles at a distance. It's fast which means it can get away easily in case it needs to retreat too. It's a really fast start to T4 if you know the enemy isn't going for the early Panzers. It can also support the SU-85 pretty well by hitting enemy tanks in the flanks or acting bait. It's an excellent unit imo, and it sits fine in T4. It would conflict with the T-34 if anything in T3.