I was in a similar situation at SIN once.
First thing to notice is that you'd have around 3.5 hrs of truly free time, 4 hr max.
Second, HKG-SIN is not such a long flight, you really should not be tired enough to need a hotel.
So....The entire SIN airport is like a lounge,

and I would be tempted to spend most of the time just loitering around.
If the need be, go take a break/shower in some lounge.
However, speaking for myself at lest, I can't imagine checking into a hotel for 3.5 hours.
How long are you in Tokyo? Would you be passing by Asakusa temple by any chance? (I need to get a very small trinket from there, if somebody is going there anyway.)
Good luck.