Or do I have to spend $$$ on a RME-Hammerfall-AEB4-O combo.

Even in the last case which sounds most elegant I am not sure which drivers or pd settings are needed.

Definitely use alsa drivers. Check the www.alsa-project.org page to see which cards are supported. I use a RME digi 96/8 which gives you 8 channel output. There are other multichannel cards that work. Ask on the alsa mailing list about other peoples experiences before buying! Some cards are easier to set up than others. I once tried to set up a box with 3 SB 128's, I got 6 channel input, but could use only one card for output without distortion. If sync between channels is important you need to get a single card that does 4 channels anyway. Or the expensive cards that allow external sync and a master clock generator, in which case a hamerfall would be cheaper.

Hope this helps

Gerard