Hi there,
Don't think the Crown Plaza is opened yet.
Last I saw it was still undergoing external paintjob and still seems to be a bit off from completion.

4 days in SIN City-
Shop and eat, vibrant nightlife along Clark Quay,happening clubs like Zouk,St James Power Station,go to the Night Safari,
Singapore Flyer ( World's largest ferris wheel apparently)
a visit to the museum, check out some shows and concerts( not too sure who or what is playing in the first week of May)
chilling out on Sentosa,
a day trip to Batam Island,Indonesia which is a short ferry ride away,
a day trip to Johore Bahru Malaysia,
a visit to the heartlands of Singapore - housing estates,MRT ride around the island,
a visit to Bugis, Jalan Sutlan, Chinatown,Little India Serangoon( not on a Sunday though,packed on Sundays)

Some may think 4 days is a tad too long in Singapore.
It may come across as a bit sterile for some people but if you plan right, check out online some of the sites and activities I mentioned above, and you will have a good time in SIN City.

Have a good trip! Cheers!

"Some people think they have firing squads in Singapore.
Singaporeans don't shoot them.. they just hang them... hahhahah"