Go for it. You can see a few things in that time.
As Dale mentioned, Orchard Road shopping strip (nothing cheap there IMHO), the Marina and Clarke Quay, Raffles, Emerald Hill road and and the hawker places all along the way. Four floors is a car crash place but geeez, it is worth a quick visit, and beware that tall ladies near the Brix at the Hyatt may not be as feminine as you might expect. These areas aren't too far away from each other so, you should be able to see a lot in short time.
Careful when it rains in the afternoon in SG. Everyone wants a taxi at the same time and everything gets slow. The MRTs have the same level of awesome as all public trains in Asia.
Just make it back to the boarding gate with 10 minutes to spare and you are winning.
Have fun....