This is how I would do it again if ever I come back to Singapore. I'm planning a leisurely trip - not one where I have to visit every tourist site in the country.
DAY 1:
Arrival at Changi Airport. Note #1) If possible, get a flight where you'll arrive in Singapore in the morning or early afternoon. This allows you to have plenty of time in exploring the exhibits around the airport. And you'll just be in time for your hotel check-in. Most of them only allow check-in at 3PM. Note #2) Get a hotel around Bugis/ Kampong Glam or Chinatown.
Go on a city tour in Chinatown and Kampong Glam. If you feel you don't have enough time, you can still go on another city tour on the last day.
DAY 2
Enjoy Sentosa. There's plenty of stuff to do there. If you have the money, you can splurge on anywhere you want to go, any activity you want to do. :) Or if you want, you can spend the whole day at Universal Studios. Just try to leave your hotel by 9PM so you can be at the theme park when it opens if you'll be qeueing for the tickets.
Or...
Go on another city tour. Visit Little India too. :)
DAY 3
Go to Clarke Quay, Merlion Park, Marina Bay Sands, and Gardens by the Bay.
Note: Days 2 and 3 are interchangeable.
DAY 4
Maybe you can scrunch some more activities here before you leave for the airport. But that's up to you so I'm leaving this blank. I'd prefer to just enjoy the hotel bed for a while. :)