Just received mine today!
I've just pulled a SID chip from an old commodore 64 computer and a second SID chip from my existing Catweasel MK3 and inserted the chips on the MK4.
Currently building a new computer at the moment and this will go into one of the free PCI slots!
Should be a useful addition when emulator software etc. fully supports it! Although the current catweasel driver software will only support one SID chip at the moment according to the instructions...