I'm surprised he didn't kill you with the hull gun.
I don't know enough about the weapons to know for sure that the Ostwinds 37mm really couldn't kill a Panver IV. All I know is that if an Ostwind gets within 1,500 yards or so of your Panzer at any angle, you are in trouble.
The 37mm on the M8 is quite useless. If you load the HE rounds, you can sometimes kill an M16 or an M3, although they'll likely kill you first with the .50 caliber MG. It goes without saying that although the 37mm HE rounds the Ostwind fires are quite effective against M3s, M16s, Panzers, and other Ostwinds, the 37mm HE round the M8 fires are entirely useless against Ostwinds and Panzers. Doesn't matter how close you are, doesn't matter what the angle is. You can't kill a Ostwind or Panzer with a 37mm HE round if you are in the M8.
On the other hand, should you decide to load the 37mm AP rounds, you may be able to kill a Panzer or Ostwind. If you are behind them at least. Oh, and inside of 600 yards. I've had some lucky disabling turrets out to 1,000 yards or so, if you hit the side or the rear of the turret. The 37mm AP round can't kill a M16 or M3 though, I've hit M3s 8-10 times with it for no damage at all.
I've always been curious as to what kind of ammo the Ostwind has. It would seem likely that it would be HE, given the fact that it IS an anti-air weapon. It also kills M16s and M3s quite handily, which would point to it being HE. It is easily the best weapon for killing any sort of building with, and since the AP rounds the Panzer and M8 have are entirely useless against buildings, this would seem to point to it being a HE round. On the other hand, an Ostwind can kill a Panzer or another Ostwind in 2-3 shots if they are close, from any angle. This would seem to indicate an AP shot, and a very high velocity one at that.
I've never heard HTC comment on it, so I've no idea what kind of round our beloved Flakpansie is shooting.