I like your query: you try to simulate a harmonic mixer at 4th LO harmonic. I replied that to my knowledge, your simulation is wrong.
Then you counter what you find at Hittite- again, they make harmonic mixers at 2x LO frequency. OK, they achieve CL of 10 dB- but guess how?
They integrate a LO amplifier in mixer chip, to optimize 2nd harmonic generation.
I do not see how it solves your problem with 4th harmonic?
I know real mixers, and my data are taken on real harmonic mixers.
You can buy from Hittite but no harmonic mixer they have runs on 4th harmonic. Your simulation is plainly wrong.