It will definitely clear the mirror box, since it does on my 3x4 RB. Infinity: hard to say. It just makes infinity on my 3x4 RB, with a home-made front standard and lensboard. The front standard is in the same plane as the stock one, but solid (not recessed) so the mounting face is about 1/4 - 3/8" further out from the film plane than stock. Then the lensboard is about 1/4", so probably total of 1/2 - 5/8" more extension than stock, if that helps. You might need a slightly recessed board, not sure what the front-standard-to-film-plane distance is on the 4x5 RB. good luck
-Ed