I cannot see why and how would you disassemble and/or move a 4 meter long cnc. I see it bolted down on a thick concrete floor in its final position and then epoxy levelled.

Grtz Bert, in your place I would have poured two layers of resin. For let's say a minimum 2% of resin curing shrinkage at your 3 mm thickness difference you've got a 0.06 mm final level error. It is not a lot but it could be avoided.