Which part of the B4's design makes you think it'd be a good idea to copy it onto a highly durable mobile chassis?
If you are talking about it being OP let me ask you how many B4 do you actually see in play at any point unless it's for trolling even if it's a 4v4 and it's behind 4 layers of defenses and AA protection? It will get nuked at a certain point, just like every other stationary piece.
As for turning it into an SPG, it would be something entirely unique to the game and to the Axis that are currently lacking compared to the Allies which have the M7 Priest and the Sexton.
There are currently 2 knock-off AVREs for the Ost, those being the Stupa and StuG III E, I don't really think the OKW needs a 170 fuel and 560 manpower slow ass turd that takes it's sweet ass time to fire with it's low range and has a projectile that either blows up on a pebble or has a 50/50 chance of actually doing some damage to something and as you can see in this topic, not even a squad wipe for that cost is just worth it if you ask me and many other people.
It's the same as the B4 currently, just a meme unit you use for the shits and giggles, nothing more.
At least turning it into the Panzer Elite Hummel from CoH would give it some practical use with the same risk and reward mechanic to it.
Bump up the price, give it a longer range and a lock down before being able to fire with a minute cooldown or something so the person pays if they're not guarding or moving it around as they should if found by the enemy and properly attacked.
At the very least that would give it a reason for the cost, speed, manual reload and it being abandoned while the crew is reloading it if under heavy attack.