I'm currently putting my RC-4 together, and so I was planning on buying Seymour Duncans, but I ended up only buying the 920D harness for it. I got a real rick bridge pickup for a good price, and I was wondering if I can still use my harness.
If not, it's not that big a loss, I can get a real Ric harness with tone circuit for like, $70.
Presumably it's this that you are talking about? http://www.ebay.com/itm/920D-Custom-...-/131744055573
The kit's neck pickup is a humbucker, whilst the 920D harness has a 250k ohm pot for the neck volume control which is more suitable for a single coil pickup. With a 250k volume pot, the neck won't have as much treble in it as it should have (which will be less than a standard Ric single coil would have anyway). So if you are any good at soldering, you could swap out the neck volume pot for a 500k audio taper/log pot and that will work OK. A real Ric harness will still have 250k volume pots for both pickups, so you'd have the same problem with that.
Or else you buy a Ric-style single coil neck pickup. The only problem with replacement pickups on these kits is finding suitable mountings, as the neck pickup pickguard hole is designed for a mini-humbucker, so you may have to make up an adapter plate. The bridge pickup is obviously routed for a J-bass style pickup, so you've presumably already sorted something for that.