Yes, neither the Duet or Uno have MIDI ports. But you can easily get a reasonably priced USB to MIDI box...
However, as you want 4ins and 4 outs... you don't mention if you need XLR's and how many... so here are a few under the 500GBP price:
Focusrite Saffire Pro 40 FireWire Audio Interface - 359.00
MOTU UltraLite mk3 FireWire Audio Interface - 395.00
PreSonus FireStudio Project 24-bit/96K Professional Recording System - 399.00
Lexicon IONIX FW810S 8-In 10-Out FireWire Audio Interface - 469.00
PreSonus Firestudio Firewire Audio Interface - 479.00
These prices are all in GBP and taken from digital village (UK retailer. The guys in the Bristol branch are very helpful and know their stuff).
There are plenty of others to choose from. Personally, I don't recommend M-Audio for audio interfaces simply because of the issues I have come across in my time as a Logic consultant.
MOTU have a very good reputation and good preamps, Presonus & Focusrite are good and the Lexicon interface sounds great - haven't used it... but reviews were very good I believe.
I think they all have MIDI In/Out except the Focusrite saffire.
Also, I've only recommended firewire interfaces as these will give better performance than USB.
Hope that helps :)