You're misunderstanding the ambush point. You aren't using the b4/ml20 to carry out the ambush, its the target to lure things through your ambushing units (tank hunter cons, AT guns, AT vehicles). That is the reason relic put the howitzer on the commander
From the patch notes where the Ml20 was added:
"Our initial testing has found that the commander lacks strong measures to draw opponents in to take advantage of the commanders ambush capabilities. In order to resolve this we are adding the ML-20 howitzer to the commander."
Unless your opponent picked a commander that has an off-map to delete howitzers, they're gonna need to send a tank back there to destroy it. Which you then ambush
Therein lies the dilemma.
If you keep the ML20 there it’s useless.
If you keep the ML20, bundle ptrs cons and light AT mines, then give it a T34/85 then the ML20 doesn’t fit this now more offensive doctrine.
If you replace ML20 with B4, doctrine is less useless, but less thematic since the B4 is a deterrent for tanks since it can 1/2 shot them with Cons/tanks in ambush to finish you off.
If you get both the B4 and the T34/85 you get a broken OP doctrine.
None of these options seem very attractive. If you wanna attract tanks you need to give them a juicy target worth diving for. Either a repair station for juicy injured tanks or a rally point of juicy injured infantry.