With the recent announcement of Recon Intel, it is now possible to get a unit with scout 4.

So far, if a unit with scout gets an additional scout keyword via the new mission from the Priority Supplies Battlefield Expansion , their scout volume would increase by 1.

For example Rebel Commandos have Scout 2. If you choose this mission where all units get Scout 1 at the beginning of the first round, the commandos get Scout 3 instead and you use the 3 speed template to perform their scout move.

But how do you manage it if these Rebel Commandos are equipped with Recon Intel? Scout 4 means twice the speed 2 template or once speed 3 and once speed1? or is it your choice?

Or is there a hardcap of 3 on keywords?

Page 46 of the rules reference guide says that Scout X can only get up to a value of 3, which makes sense given that there's no speed 4 movement tool. You should check the Rules Reference for this type of question, it's pretty good at being cut and dry about this type of stuff. FFG is doing a decent job keeping the rules for this game pretty straight.