Welcome on the Beoworld Forum. As far as I remember, the PLAY button should activate the last used source. Does volume control work?
Pause N.MUSIC by pressing STOP on your Beo4 - press PLAY on the IR eye. Does it work?
Next: your BL active must be set to L.OPT 5 - only audio sources, or 6 (if there are video sources in your setup).
I can test the PLAY button tonight when I`m home, I have a BL actve and BS5 too.
Stefan