With the 4mm cutter that would essentially give you only 4mm uncut on either side of the mortise (1/2" = 12.66mm). Anything less than that and you would be sacrificing the integrity of the piece. In any case, the fence won't go lower than about 4-5mm from the top of the cutter, and the distance from the base to the middle of the cutter is already 10mm. The only way you could do a piece thinner than 1/2" would be to elevate it slightly off the worksurface and go at it without the fence being lowered on the Domino.
What were you thinking of doing that's less than 1/2"?