There is no easy way to do this in the current version of VASSAL. The next version (3.2) will have features that make using maths much simpler.
The way I would approach it is to maintain the current round number in a Global or Dynamic Property (GP/DP) and have the layer trait ‘follow’ the value of the Property. Lets call this property ROUND
This means the layer trait is just displaying the value of the ROUND, not generating it. All manipulation of the round number is now done by manipulating the value of the ROUND GP/DP. I find this much easier to handle when building the module.
The simplest way of maintaining the ROUND+3 value is then to just maintain a second GP/DP (say called ROUND3) that contains this value. ROUND3 would be initialised to 4 instead of 1 and whatever actions you take to change the ROUND DP/GP, you also do to the ROUND3 DP/GP to keep it in step.