I recall this being beaten about a few months ago but do not recall the conclusion.
When I got my Rousseau plate mounted and the Amana tongue and groove bit to make a test I see that it is going to make a 1/4' groove. Well if the 4mm plywood was 6mm then I would be set. Plan B which is a bit more pure is to resaw some wood on the $150 20 year old nearly new 14" Delta bandsaw. If I buy 4/4 oak which is really 4/4 can I get 3 slices? My planer works fairly well so I should be able to make it pretty enough.
Or does someone in Atlanta sell 6 mm or 1/4" oak plywood? Actually resawing will frustrate/make me happy as it is a bit more pure but I have not done any resawing yet.
Shimming 2 mm will not be an acceptable answer. I am not keen on using the table saw to make the tongues 4 mm but that could be an option.