Yes you are underpaid compared to singaporeans but thats the average pay that is offered to malaysians.

The main reason is that singapore gov mandates that any job has to be advertised to singaporeans for 6 months prior being opened up to non singaporeans. All the higher paying jobs have been taken up and these are the ones with less attractive pay therefore it gets opened up to non singaporeans.

Unless you are from nus or ntu. 2800 is the average fresh grad / 1 year experience engineer in singapore for malaysians. But one thing to note is 2800 in singapore has much more cash value compared to 4k rm