Anytime appearing for all the 4 modules together would be better as you get the additional benefit of inter group marks set off however assuming that time is limiting factor for you as you are working my suggestion would be to appear for Module-1 and Module-3 and Module-2 and Module-4 thereafter. In this the advantage you get is two days of leave after one Module ( 2 papers).
Module-3 would be advisable to be completed as soon as possible considering there's a possibility of DTC coming into effect which would lead to changes in the Taxation paper.
Please note I am not a CS student and don't have exact idea about this course. I had given this advice to a close friend of mine who is pursuing the CS course and after giving a thought to what I said my friend told me my strategy was good for him as he had the same time constraint.
Our experienced members in CACLUBINDIA would be able to give you proper advice.