Ted,

It's tough to say what the real cause of your upset was, but goodness
knows we're going to try.

Your tow pilot makes a good point, and one you should take to heart.
Before your next take off, note a physical feature on the runway where
the tow plane starts its take off roll. Most tow pilots apply full
power immediately. Since the tow plane is moving slowly at this point,
it is generating a good bit of turbulence that you are going to have
to negotiate at less than flying speed and probably well below a speed
where your controls will have anything close to full authority. Tail
draggers like the B4 are especially prone to upset at this point. As I
roll toward the tow plane start point, I try to anticipate the upset
(almost always a drop of the right wing) and catch it as it starts
rather than letting it catch me unawares.

The B4 has an especially strong elevator. If you get out of sorts, you
have to be extra careful not to over control. PIOs are common in the
model, especially during the first few aerotows.