General consensus is, T4 is perfect for con upgrade as:
-provides alternative option to rushing T-70(not really atm, but theoretically)
-still can arrive relatively fast
-doesn't impact early game, where cons are ok-ish
As a Soviet player I always dreaded cons vs okw, since around the 5 min mark every volk squad has STGs and without ppsh on every cons there is no contest. I fear this upgrade would come too late at T4. Always talking regarding 1v1s.