Well, mostly B-4 are lost to combined arms pushes of infantry and medium armor.
There fixed it for you. The B-4 is not ONLY countered by Stuka strike, but it is the easiest way.
Doing a focused push will get you close enough so you can fire at the thing. Unlike ISU/Jagd it cannot retreat and it has much less HP and armour.
Furthermore I've lost a plurality of B-4s to flanks by tanks not to Stuka.
PS. About it oneshotting full health JTs at 3 vet. Ive seen that happen, not alot, like once or twice, don't know if there is some critical hit with hitting the rear armour or some random event, but I've seen it. And didn't use mark target. I hate it when people put mark target on units that I fire at. Because the only thing that is going to happen is that tank moving and my shot missing, mark target and precision strike down the drain.