I have seen crashes using Magic Bullet Denoiser in Premiere Pro CS5. Red Giant says Denoiser is currently unsupported for CS5, and I assume that goes the same for CS4. The Denoiser compatibility notes are here:
Unfortunately, I don't have compatibility info on Magic Bullet Steady for CS4. I know it is discontinued for CS5. Perhaps you could contact Red Giant directly to ask them. Even better, if you could post back your results for other users who may have the same question.