meh. your going to want to back the gain down, or make the box a little smaller than recommended. I used 3 different set ups with level 4s and they didnt like more that 1500rms in spec box. Thermally they can handle more like 2k, but mechanically is what you want to watch when throwing double rms at them. But what your asking, a 3500 on 2 4s would be fine, just let them loosen up then adjust accordingly. I ran 1500 to one of my level 4s for about 6 months and could never get it hot, but if i wanted to bottom it out, i could.
i wouldnt worry bout em, ive ran my share of DCs;) including lvl4s infact i had 2 lvl4 12s and 15s on almost 4k for bout 6months. then i went to xl 15s ona 5k then a single lvl5 18 on a 5k at .5 wit no issues what so ever from any dcs iv ownd and every single dc iv owned iv ran any where from 1000-2000 over the rms ratings. my xl 15s were in 4cubes each wit over 2500clamped watts goin to em.
op like i said youll be fine wit 2 lvl4 18s on a 3500. me personally id drop the bish to .5 and let em eat youll be happy wit em.