If you want to be sneaky, you can always lie. Your potential employer can't demand your salary slip or payment records. They can only ask you nicely.

You could be earning like 3k, told them you earn 3.5k and demand 4k. However, if the employer was only prepared to give 3.5k, you might actually not get the RM 500 increase because you wanted a 1k increase and you will be back at square 1.

Alternatively, you could just tell them you are getting 3k but you want 4k. Most likely you will not get the 4k offer. If they offered you 3.5k, you would most proabaly take it. Unless you are brave enough to say no, you won't take it and they are desperate enough in getting you at all cost.

I think 20% is a reasonable standard. Anything less than that is generally not a very good idea. So 20% X change job 5 times = at least double your current pay.