Having googled it a bit now, it's fully possible I have my wires crossed, since I know that P4 machine had the SiS 645 chipset which of course had built in audio.

I definitely used the Sound Blaster with my 486DX100, and I recalled migrating it to at least one other machine after that; it was nice for the joystick port and also the better wavetable synth on classic games.