You've picked a couple of extremes here, that's for sure.

RPM's don't enter into the issue. It's all strictly about angle of attack. For a prop that's not traveling forward the angle of attack is given by the local angle of the prop blade. In reality it's a trifle more complex than that because the prop tends to create it's own airflow with the air being drawn into the disc on the front and pushed away to the rear so the angles are somewhat mixed up. But basically a low pitched prop will not be stalled and a high pitched one will if there is no forward speed of your model.

Basically if the 15x4 isn't stalled at 1500rpm it won't be stalled at 15,000 either. At 50 rpms the 15x15 SHOULD be stalled but low speeds of that nature can do some funny things sometimes. But ignore that example. Certainly I think it's safe to say that by 1500 rpm it would be stalled and stay that way regardless of the rpms.

But all this is for static operation at zero or very low airspeeds. As the airspeed rises at some point the 15x15 will become unstalled because the air is moving into the prop disc faster and the angle of attack seen by the blades will become less. Assuming the drag of the airplane and the engine power will let the prop reach that speed. If it can wind up and the prop is operating well the final speed of the model will be very high for a given rpms from the engine. Meanwhile the 15x4 will accelerate very fast thanks to it's low pitch but it'll top out it's speed at a much lower value assuming the engine is turning the same rpms. But all this is ignoring the fact that to turn these props at the same rpms will take probably someting like 6 to 8 times the power for the 15x15.

But a stalled prop still produces thrust. It just can't spin as fast as the engine would like because the extra drag of the stalled blades prevents the engine from revving up to it's peak power rpms. This is why fixed pitch racing full sized and model aircraft often take a lap or a bit of time and distance to come up to full speed.