Obviously, the 2.4 GHz itself cannot enter the LNB due to waveguide cutoff. 4x2.4 GHz = 9.6 GHz, which is a theoretical possibility.
But assuming you stick to the legal power limit on 13cm and drive the final amplifier not too recklessly, the harmonics should be far below any danger limits for the LNB. You want to use a 4 W amplifier presumably for SSB so you would need to back off a bit anyways. And then, I don't think harmonics are a problem.