First of all, you have a strong, solid amplifier in a Concert Bass. Second of all, the Concert is built to handle one or two 4 ohm cabs. It's a good choice!
Regarding cabinets, a lot depends on the kind of sound you want, TOR. If you want a full, boomy sound, a folded horn cabinet such as a 118MH is just the ticket. If you want something with a little more range and articulation, a 215 is a great cabinet. (My personal favorite, at least.) While many players today like the sounds of 410 bass cabinets, which only came out with the Sunn brand after Fender bought the company, I haven't heard any I liked other than Ampeg's SVT-410HLF and PR-410HLF. My recommendation is to look for a 215; be sure to get a 4 ohm cab not an 8 ohm.
Here's a really helpful site you might check out, it consolidates listings on Ebay and Craigslist nationwide, so maybe you can find one within driving distance of your home:
http://www.jaxed.com/cgi-bin/mash.cgi?itm=sunn&cat=mus.
Happy hunting!
