As for monitor controller I'm using my SSL X-Desk for that, but there are a load of worthwhile stuff. From about $200 to the moon. The Audient Nero has been getting good reviews.
The Apollo X4 would probably fit your needs well right now, but it's a lot of money for latency free monitoring (the you can put very good dynamics/effects on though) and I would save up to a Apollo X8, X8p or similar because then we are talking completely different beasts and the flexibility increases ten fold.
As for low latency monitoring, for that to work make sure to monitor without hitting the DAW first. Doesn't matter how fast a computer you get, if it has to go back and forth between the IO and the DAW it's going to be too slow and getting below 10 ms is pretty much impossible (due to drivers etc).
As for thunderbolt vs USB it makes a difference, but it is going to be super small with only 4 channels in and 2 out... it starts to make a real difference when you are sending 32 channels both in and out or doing DSP processing on the interface. Just make sure to not connect lots of other stuff to the same USB bus.