Zis, Su76?
You can get both wayyyy before OKW P4 hits the field.
Yes but the okw can rely on the puma to deal with any early t-70's . as for the su76 the stug flat out curb stomps it unless it can use its longer range to its advantage.
It's not really a risk. Soviet T3 offers everything you need:
M5 for AI, T-70 for AI (and against other light vehicles) and SU-76 for AT and light artillery support.
Its a well balanced package no doubt but ost tier 3 is still stronger. get an ostwind out to deal with the m5/t70 first and once he follows with a su76 nail him with the stugs. also if you are really desperate 2 222 can also effective fight t70's
Panther is faster than any medium tank, while also having blitz. Will consider this a brainfart.
the blitz on both the panther and tiger got nerfed into the ground providing only a small speed increase.
A panther is first of all not a true hevy. It is substantially cheaper, whilst retaining the advantages of speed and high armor. A better comparison would be a panther to 3-4 t34s. A panther works in tandem with support units. It tanks damage, whilst supporting units dish out damage. The stugs on the other hand, require meatshields. They are dps, not true armor.
2 problems with this logic. your not going to have enough resources getting both panthers and supportive units unless you mean paks or shreks. which are rather frail and unsuitable for offensive actions. high dps can be just as great a defensive tools. high damage means death stuff , death stuff means no need for defence. if you need a breakthrough unit you will not find it in tier 4 but in the tiger or the new and improved lefh.