Singapore is definitely a must visit place in Asia because of the list of things that you can do there that can't be done elsewhere in Asia. Although there is an extensive list of places to visit and things to do in SG, 4 full days in the City will be enough to make the most out of your visit. You also have an option to visit Malaysia (KL) but you will have to stay longer (5-6 days) to make up for the long travel time.
How to start with the Itinerary?
First, list all the places that you want to visit and all the things to do in SG because you can accomplish everything in just 4 days! Just cross out the things later on that you decide not to do once you're in Singapore.
Second, decide if you'll be going to Malaysia specifically in Johor Bahru (nearest City to Singapore), or if you'll go further to Kuala Lumpur or other cities in Malaysia.
Lastly, Singapore is a very small place, so just make sure that the places you'll visit are nearby since this will save a lot of time despite having efficient transportation systems.
Below are my recommendations for Itineraries going to Singapore. Feel free to alter, combine or copy depending on what you would like to prioritize, your budget, or the people you're with.
If you're going to KL, I suggest you just spend 3 full days in SG based on the list above, and spend additional 2 days in Malaysia.
Travel time to Kuala Lumpur from Singapore should be around 4-5 hours including immigration and bus ride for one way. But 1 day will be enough to go to key tourist spots in KL.
The two Itinerary above can be accomplished as long as you're efficient and do not have a lot of resting time. But you may opt to remove some of the above based on your preference. For Example, we removed Night Safari because we didn't want to try based on reviews that we heard and we skipped Bugis Street because we were too tired to walk hehe :)