IIRC most ppl here with a FUBAR have the regular IV (mainly due to some outstanding deals that were available).
The IV can only handle up to 24/96 hi rez, whilst the dac in the plus processes up to 24/192, but there isn't a hell of a lot available to buy at that resolution at the moment. I believe that the USB is limited to 24/96 on both of the anyway (and the USB uses a separate dac chip? SOmeone more knowledgeable might be able to correct/confirm this).
I recall seeing a review somewhere where the reviewer thought the regular FUBAR IV with the separate power supply gave better sound quality than the IV plus, and worked out a bit cheaper.
I find it a great compact head-fi solution, but I'm not sure how it rates just using it just as a dac.