I've used the Plaza Premium Lounge in KUL before. You can book with them directly, but if you search around online there are various websites that sell access, and you'd probably be looking at about $30/person.
In terms of the lounge, it was ok but nothing stunning. There are showers available (they'll lend you a towel), but they weren't in great condition (think slightly run-down hotel rather than a high-end airport lounge), though it's enough to let you get clean and refreshed! There was some food available, but not a great choice, and some drinks. Free fast wifi though, newspapers and magazines, and fairly nice chairs. For me it was worth the discounted price I'd paid, but not the full price. If you have any fancy credit cards it's worth checking with the issuer in advance, as some will include access to the lounge (the people in front of me in the queue had free access with their platinum card for example)
I don't believe there are any other pay-for lounges there - the rest tend to be airline ones reserved for their frequent travellers and business/first passengers.
That said, KUL is a very nice airport to just wait around in. On the upper level they have 4 areas where you can sit to watch TV, they have free wifi, they have a small tropical garden in the centre of the airport, and everyone's really nice. Depending on the flight times, you might find you only want a lounge (for a shower and some food) in one direction, and in the other just relaxing in the airport itself will be fine.
4-5 hours is possibly too tight to make it into the city and back (I've not tried it though!), hopefully someone else can comment on that!