high or low-mount? high-mounts require the removal of the emissions cannister do-hicky under the right-side cover. if that's the case, do a search on "smog removal" or "emissions cannister" and you'll see several threads on how to pull this thing off.
if they're low-mounts, i'm not aware of any fitment issues from '02 or '03 cans on '04 bikes. for that matter, all low-mount cans at least should fit on all monsters from all years. same header-diameter and same mount orientation.
if they're low-mounts and don't fit, you may want to double-check that they're for the monster and not supersport.
Low,just the diameters are the same,from the pipe to the tube of the can, they don't slide into each over,and the holes for the oe bracket are not at same spot as the stock ones,
that's ok i can make bracket,but i don't want to enlarge the
holes from the pipes...