Can anyone tell me who sells a stage four chip for a Nissan Altima v/6 ?

Answer 1

No such thing. Stages are developed by companies usually as a quick way of saying a more extensive upgrade kit for car mods. For example "Stage 2 turbo kit, Stage 3 etc." There is not much more you can do to a 'chip' beyond adjusting air/fuel ratio without drastically modifying your engine. And most all "Stage 4 kits would come with the chip already."