Don't forget that an amp that delivers 50 watts into 8 ohms will deliver at least 100 watts into 4 ohms... if your speaker is 40 watts at 8 ohms and your amp is 50 watts at 4 ohms, then you some 'headroom' left... the amp will only be delivering around 25 watts.

Ohms law, you see... although it doesn't apply rigidly to amplifier output (there are other considerations such as Total Harmonic Distortion, etc).