Hello,
at the risk of telling you how to suck eggs, did you know you can check in with Singapore Airlines up to 48 hours before departure?
This is what we did last November; you can check in on the internet, which includes changing your seats if there are others available.
As for the 4 hours before-hand at Heathrow, yes, our tickets said the same and we were there in the queue 4 hours beforehand. However,the check-in desk didn't open until then! So to be pedantic everyone on the flight checked in luggage less than 4 hours before take-off!
Cheers,
and good luck,
John.