Do I need a re-entry permit if I have a valid work permit and an extension of stay stamp while traveling outside Thailand?

You are confusing two completely different things.

You hold a singapore passport that lets you enter the country without a visa for 30 days (just like people from 50 other countries can)

BUT

IF you hold a valid extension of stay (which you say is good until the end of the year) you NEED a re-entry permit to come back into the country and get stamped back in on it until it's expiration date.

So IF you're gonna be still working for the company who sponsored your current extension when you come back in April GET a single re-entry permit before you leave to keep that extension 'alive'

15-20 minutes max to get a re-entry permit at Suvarnabhumi in the International Departure #2 area. Follow the signs

If you keep on working, you need a re-entry permit to keep your Extension of Stay a live,

If you stopped working you need to cancel this Extension of Stay and work permit.