Those MC4 branch connectors that smkettner posted are nice. They are available in parallel and serial type. I used them in my installation, and they worked great. I just strip-tied them behind the panels. No need to mess with a junction box. I used 10AWG fine-stranded exterior grade wire. (Here are some pictures of my installation) In your case, if joining four 24V panels in series and parallel, as smkettner suggested, you'd have 48V out to charge controller. I believe that would give you about 15A current (max) output. That should result in less than 1% voltage drop for 30 foot run of 10AWG wire, if my figuring is correct. I don't see why you would need larger wire from panels (or junction box, if you use one) to charge controller. Do use larger cable from charge controller to batteries.