Cutting 0.004in from the head is fine. The problem, as Kebowers properly points out, is whether they are cutting it to make it smooth from surfaces imperfections (like corrosion)or from warpage. The 0.004in cut is about the limit if it was cut for warpage ( maybe too much, I would like less than .003).
Be absolutely sure a new Mercedes-Benz head gasket is used. They have improved the gasket where it leaks at the front cover. None of the OE gasket makers were using this gasket when I checked recently.
We do many of those head gaskets mostly for the front cover leaks. We never do a valve job. Those motors are going to go well over 200k before valves or guides start having problems. There is no sense in the extra money and the waste of good metal, cutting anything at 100k.
Bosch Master, ASE Master, L1
33 years MB technician