I would want to be drilling at least every 2 inches to ensure good wet out. I'd probably do 40mm to be safe.
You don't need to drill though, you can just punch a hole with a screw driver or anything. For 4sqm, I'd probably get a plank if wood and put some nails through it along its length spaced 40mm apart. Then you can punch 10-20 holes at a time without having to measure spacing.