Only you can answer that wesk702...
What is it you really want to achieve in the end? - what items are you using the existing mixer for?
Personally, I'd identify your end goal and then map the progressive steps and prioritize them [usually based on budget] and work toward them.
Audio work is more like alchemy, to me - part science, part art - you miss either part and the magic is gone.
If you are determined to scrap your existing mixer, then yes you could purchase a new USB one [while I have no reported issue using a Thunderbolt > FireWire cable/adaptor - I personally would not do it for audio (to much digital signal conversion for my liking)]. While I have not used nor own it I have heard some good feedback on Allen & Heath ZED gear - so maybe ZED-10. Could be found on ebay for $150-$250 - it is only USB 1.1 so you could be limited on audio thru put if you try and do much recording thru it & I believe you'll may need to toggle a setting on the ZED for USB to be output vs input on the ZED itself.
Audio gear is kind of like owning a boat, you can always... ALWAYS... throw more money into it.
Personally, I would save up the money [the extra $100] and just get the Focusrite Scarelett 18i8 <ebay range $250-350>. It's a clean well respected 'low end' audio interface, and allows you to continue using the gear you have & know. Yes it may have more additional XLR interfaces than you need currently, but get's you the four 1/4" inputs you are looking for, with not needing to install third party drivers [I use it in Maverick/GarageBand using the OSX default USB drivers].
In the end, it's all about what you want to do & how much you want to spend.
R_