4mm for the whole thing.

i've clocked it at 96 mph with an OS46FX and 9x9 APC but its heavy cuz I had to use 1/4" ply for the ribs. Penetrates well though so the I guess it's a good trade off. I would think if you use fins that big that flutter will be a major cause of its (maiden flight) crash. Mine are only 6 inches tall. Rock solid at speed. It will also fly quite sedately at full trim throttle (trim full on, throttle full back).

Great design modification that Tattoo came up with.

edit--> I just looked at your plans for it. I think (you can always try it) that the elevons are too big (mine are only 1 inch) and the fins are way too big. With my 1 inch elevons, I only have 1/4 inch up, 1/4 inch down. And I've still bent the "delta" part at the trailing edge just in front of the elevons going into high speed dive/pullouts.

Check out the spad site for the ddust plans and you'll find two size deltas--the one I used and one for the engine you have (the one I used I just scaled up to the 46 size). You'll find that my bird utilizes a "fold-over" design to construct the main body. Your design will fly (single sheet) as one of my buddies has built and flown one like that. His was an experiment. Mine was a proven performer.

Either works. Just either modify yours or use the other plan set.