The difference is 2. Seriously, they are similar, (now quoting the Jorgenson steel book)"except that 304 is modified by lowered carbon content. This provides good resistance to corrosion in welded construction where subsequent heat treatment is not practible."
They are 18-8 chromium-nickel with the 302 having .12 max percent carbon, with the 304 having .08 max percent carbon. Everything else being the same.