Question: To be clear I'm not looking for any myself, but I had thought there were no PS4 cheats and yesterday a friend and I were playing, and a player was griefing us. We shot him endlessly with rockets, grenades, mines, you name it... no effect. I even hit his avatar directly (so not in a vehicle) with an RPG and he went flying, then simply stood up and killed me. Seems like this had to be a cheat... no?

I would like to actually be linked to an authoritative source stating that PS4 cheats (serious cheats, not just silly cosmetic mods or whatever) do in fact exist currently in GTA Online for PS4. I haven't been able to find any information that wasn't either really old, or for Story Mode.

Answer: 
Yes, it was a cheat. More specifically it was a cheat as a result of a mod, there are normally no cheats in the game. No secret passwords or konami-code type sequence of button presses that will enable god-mode.

Modding on consoles is more uncommon than PC, because it usually requires altering the console's hardware in order to run otherwise unauthorized programs or altered versions of the game software. This tends to get more common as the console ages, as new quicker/easier methods of modding are developed, and as games stop getting updates to combat the cheaters. The XB360/PS3 versions of GTA Online tend to have more people cheating than the XBOne/PS4.

If you encounter a player like this, your best option is to report them, and then switch to a different lobby.
If you are on the PS4, it was most likely a godmode exploit, as modding was made incredibly difficult on this generation of consoles, as compared to the Ps3/Xbox360. An example of an exploit that worked on patch 1.50 is [this one](https://www.youtube.com/watch?v=rjKRYO50Wfo).