Presuming that you are able to check-in online for your onward flight, and given that you do not have checked luggage you should NOT need to clear immigration, although you will need to go through security.
There is an airside bus that runs from Terminal 2, gate 51 to Terminal 4. The bus runs every 15-30 minutes, and takes around 12 minutes. In order to catch this bus you will require a boarding pass for your flight from T4.
Presuming your inbound flight is on time you will have no problems getting between flights in 3 hours. Of course if your inbound flight is delayed then it could be a different story.
For completeness, if you did have checked bags then you would NOT be able to take this bus - unless your bags had been checked through to the final destination (which they would not be on a flight connecting to AirAsia). In that case you would need to clear immigration in T2, collect your bags, catch the land-side bus from T2 to T4, check your bags with AirAsia, go through security and immigration (in that order - T4 is different to the other terminals at SIN where it's reversed), and then go to your gate. This should be doable in 3 hours, but will be much tighter than without checked bags - and if your inbound flight is delayed would become a problem much quicker!
If you were not able to check-in online for your AirAsia flight (but still had no bags) the it would be the same path as above, only obviously without the need to collect bags.