The commander’s theme is “tank ambush”. This is achieved by deception and heavy damaging abilities that rely on such deception.
More specifically, you have PTRS conscripts which have camouflage and can saturate areas with ultra cheap 10muni stun mines. Tanks and tank destroyers also get camouflage to position themselves to strike at enemy vehicles, either by ramming (76) or shooting (su85). The damaging abilities are the conscript grenade assault, T34s coming out of camouflage to flank and the carpet bombing ability.
Here we come to the ML20 or your B4 proposal. You cannot know exactly where an enemy tank would hit a mine allowing you to pounce, hence you’d have to reposition the B4 to actually get it to fire, which is a 15seconds minimum process from starting to use the B4 to actually landing a hit. Most engagements are over way before that. Thus the ML20/B4 does not fit the core of the commander’s concept since it takes too long to deliver a hit. Now, if the B4 had a wider firing arc you could use it defensively with the direct hit ability, but as it stands you”d have to reposition it to get it to fire, which would take too much time and your opponent knowing you have a B4 would avoid close proximity, making it harder for you to utilize the ambush part of the commander since he’ll be very cautious about getting close to the B4.
Now contrast this to the T34/85, a tank that can beat a Panzer IV J 1v1 and can brawl with a Panther with favourable RNG. Such a tank would benefit immensely from being able to ambush enemy armour that’s been stunned or grenade assaulted. It would provide a huge late game lift to this commander by replacing the T34/76 as the main power unit in your late game composition, while fitting both commander themes perfectly, being able to ram, camouflage and flank. The commander’s theme is “tank ambush”. This is achieved by deception and heavy damaging abilities that rely on such deception.
I agree that the ML20 doesn't fit from a gameplay perspective so no argument there. The B4 however does in my opinion. Who said you need mines to hit/kill tanks with the B4? B4 tank killing is all about anticipation. Most people use their tanks static in combat and this goes double for casemate tanks so hitting is not a hard as people think. This only becomes easier as game goes on and it acquires vet. People also tend to be predictable where they repair their tanks since its usually in the same place, especially for OKW mechanized building. So again I don't agree with the notion that it doesn't fit since it can provide the doctrine with some much needed lategame firepower to counter axis super heavies. But if it is deemed too weird to be in this commander I would ultimately understand.
I still think that the M4C would fit better then the T34-85 if you absolutely need a premium medium. It can buff nearby tanks with radio net, a bonus that should not be under estimated and it has more at power then the 85 due to the HVAP rounds. It would also be able to camouflage like the other tanks of course.
Besides putting the 85 in another commander would be too way much, if you really need ram you can always get a 76. This game stay interesting due to the different strategies and tactics a person can use. Giving too much of the same tools to commanders breaks the most interesting part of game.