150A. Fuse for the max rating of the wire, which for 4g OFC is usually 150A. If you start popping fuses, swap out the 4g for some 1/0 and fuse for 250-300A, depending if you buy CCA or OFC
what wire do you have, 4 gauge in car audio is too broad of a sweep because of the variations of wire. generic no name usually are fused @ 60a, good stuff like Kicker Hyperflex gets fused @ 100a.
I just took a look at the terminals, I REALLY think you could squeeze 1/0 in there. Make an excellent ground for it, and fuse for (2000watts/13volts)=~150amps.