I’ve got a 4000 sized spinning reel that I’d like to make a catfish and occasional saltwater setup. The catfish won’t be huge - I’d guess 10-15 pounds max. I just want something to throw way out in the pond while I’m bluegill fishing with my kids. I’d also like it to be useable for very occasional saltwater inshore fishing (once or twice a year) if possible. The reel is rated for saltwater use. 
 

What length/power rod would be the most versatile for this setup? Also what kind of line (Type and test) would you suggest?

I would get a 7' to 7'6" med hvy or hvy spinning rod.

I would probably use 30 pound braid in a bright green or yellow.

I use a 7' medium Ugly Stick for my channel cat rods but if you're going to be casting for saltwater fish you might want something a little nicer. I use 20lb braid on mine but you could probably step up to 30lb if you wanted since you're fishing the salt also.

A decent size 4000 spinning reel should be more than enough for most catfish you will encounter. That is the size I use and I have caught flathead, blue, channel, white, and other types of catfish. A 6'0''-7'0'' rod medium heavy to heavy action would be a good choice for a size 4000 spinning reel. 30 pound test braid or 15-20 pound test mono is what I mostly use and will use a stronger line if needed. 

I use regular 7’ MH inshore graphite rod which is a lot lighter than regular catfish rod but got decent backbone on both spinning and baitcaster set up for catfish. I use 20lb braid on spinning and 15lb mono on baitcaster.