I am right there with you tyroflyer! Would love to have a proper Pz IV suitable for the invasion of Poland in 1939.
The difference being, with the BA armored cards, they already have the turret for the BA-3 and BA-6. Looks like they could add 12 tires to the T-26 sprue (along with that turret). So they may be one "body" sprue away from having a kit that does double-duty (if they allow for the slight differences in those three locations. One sprue, and they have a kit that serves for two vehicles.
The early Pz IVs are a bit more problematic. The same hull would mostly do for all four early models (Pz IV A thru D). But there are enough differences that they would likely need multiple sprues to cover the turret and superstructure changes:
Pz IV A = Angled front plate for driver and gunner. wider superstructure, two-piece hatches for driver and gunner, circular bullet-splash ring around turret base, rectangular ports on turret front and rear,
Px IV B = Straight front plate, no MG, narrower superstructure, single-piece hatches for driver and gunner, angular bullet splash ring around turret base, rectangular ports on turret rear, circular ports on turret front and rear, cupola changes
Pz IV C = Straight front plate, no MG, narrower superstructure, single-piece hatches for driver and gunner, angular bullet splash ring around turret base, circular ports on turret front and rear,
Pz IV D = angled front plate, with MG, narrower superstructure, single-piece hatches for driver and gunner, angular bullet splash ring around turret base, circular ports on turret front and rear, gun mantlet changes, rear deck side changes
Looks to me like it would take three different turrets (one for A, one for B and C, and a third for D models). Also three different front plates (again, one for A, one for B and C, and a third for D models). Probably two different muffler-area configurations (one for A, one for B, C and D models), three different sets of track guards (one for A, one for B and C, and one for D models).
And as I look back over having typed up all of that (after a close examination of the blueprints and notes I have taken of the first four models), I am struck by the idea that the B and C models were very, very close in appearance. The A has different track guards, a wider superstructure, a different enough turret (hatches, gun ports and cupola) to require its own, and a slightly different muffler area.
So I can easily see making a B/C kit easy. I can even see making a B/C and D kit. But to make an A/B/C/D kit would require a lot of effort, with many sprues to cover all the changed bits that the Model A has all by itself.
I would be super happy to have a B/C kit. Both models served in Poland (as did the A). The D did not, but it served in France (as did the A, B and C models). In terms of numbers made, we are looking at 35 As, 42 Bs, 134 Cs and 232 Ds.
It was the Pz IV Model D that had 48 converted to Tauchpanzers (submersible) versions, for the planned invasion of England. Also, many of these early models (A, B, C and D) were later converted (back-fitted) with improved weaponry, and sent off to places like North Africa. So in terms of optional pieces, I can see the sprues needing different turret weapons thus increasing the usability of the kit.
Buy one and build for Poland, another to paint for France, and a third to serve in North Africa. All from one kit!