A lot of answers can also be found here:
http://www.flyertalk.com/forum/milea...es-please.html
From personal experience, the Transit Hotel is nice but you won't get your money's worth during a 4-5 hour turnaround. You can buy just a shower at the Transit Lounge (next door to the hotel entrance) for $16 SGD.
All airline lounges in SIN are for departure only - it doesn't matter what class you were in for your arrival. You'll need a hard copy printed BP for your same-day departure flight to gain access. Print your BP out before you land (in HKG or NRT) and you won't have to stop at the transit desk. (transit desk also opens at 3am so if you don't have your onward BP already then you're out of luck getting into any of the lounges before then).
The SQ Kris Flyer Gold lounge is the only *G lounge that is open 24 hours. Food is good but limited and there are spots where you can nap but it is small and there are no showers.
The SQ Silver Kris lounge is only for *G on Biz/First class. There are showers and the food is better.
The SATS lounge (next door to the SQ entrance) is for *G members in any class but it only opens at 3am. The food is decent and there are massage chairs in the back.
There are lots of things to occupy your time in SIN. It's a huge airport and everything is connected airside. Unless you really need a 2-3 hour nap in an actual bed with no sound/light, then skip the Transit Hotel.