0.004" won't hurt anything. The most important thing, accuracy wise, is that the brass is trimmed to the same length.

I shoot 7mm Rem Mag brass resized and fireformed into 257 Weatherby Mag with no problems and excellent accuracy. The 7mm Rem Mag brass is 2.500" max length, with the trim-to length 2.490". The 257 Weatherby Mag max case length is 2.549", with the trim-to length 2.539". When the 7mm Rem Mag is necked down in (minimally) fireformed, the cases are roughly 0.04" shorter in the neck than Weatherby cases and specs. If you compare them side-by-side, the reformed 7mm Rem Mag brass are noticeably shorter in the neck (the 257 Wby has a long case neck, however). My rifle shot the resized 7mm Rem Mag brass into consistant 3/4 MOA groups during fireforming. So no, the small difference doesn't really matter, as long as they are not over the max case length spec, and they are trimmed to a uniform length.

Mike