Why does a 4-prong plug melt in my Honda generator?

It happened twice already. I use this EP6500CSX generator as a backup, when the solar panels don't get enough light, and the turbine doesn't get enough water at the river. Is it bcuz the inverter spews some power back into the generator?

It is because you are CLUELESS about how to wire ANYTHING. Even if we were to ask specific questions about what you have done to date, YOU wouldn't know how to give answers that would allow a proper diagnosis without an ON SITE inspection.

Note: if you HAVE an inverter and a generator on the same circuit at the same time, THAT is proof you NEED a professional to fix what you have done wrong.

You're trying to connect the inverter in parallel with the generator? You can't do that. You don't have any circuitry to keep both devices in perfect synchrony, 60 Hz, phase locked, and exactly the same voltage.

You're lucky that all you did was melt a connector.

If its melting something is too hot