Now with latest spy photos available I was able to examine the prototypes closer. This prototype is definitely 4er based. The front is identical, the side character lines are identical, the rear is almost identical. The car seats low, it has low roof line, greenohouse is very coupeish. It's a 4dr coupe / 5dr fastback. And especially the D-pillar & extra side window after the C-pillar indicates the 5dr layout. And the only BMW cars with such layout are Tourings, SAVs, SACs & GT. Sedans, Coupes & Gran Coupe have the kink in C-pillar.
And as said: with 2er Gran Tourer / Active Tourer trademark registered by BMW (indicating a coupeish fastback), it seems 4er Gran Tourer / Active Tourer is possible. So 2er & 4er will offer a Gran Tourer / Active Tourer instead of true 4dr coupe ala 6er Gran Coupe. And with 5dr fastback available, there won't be a Gran Touring shooting brake at all in 2er & 4er model line. While there will be one along 6er GC.
Comparing 3er GT to 3er Sedan and to 4er GT, it seems 4er GT will be less practical & sportier than both other models, while 3er GT will be more comfortable & spacious than sedan & the 4er GT.
Sure 4er GT is in this stage of development already since it's just a derivate of 4er coupe. Canceling 3er GT or revising the car so heavily just ahead of FEP testing would be a fiasco of epic proportions, and heads would roll.
I'm now sure both cars are coming. This one, and the 3er GT as well (in 5er GT style: elevated sitting position, hatchback, roomier etc).
But I agree it's weird. I didn't expect that coming: a 5dr 4er.