Nope
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of course not
it happened when ss4 Gogeta fused with ss1Go-tanks and Gohan
No, and speaking canonically there was not even a Super Saiyan 4 Goku. Anything above Super Saiyan 4 is fan made without the influence of Akira Toriyama in any direct way, however Dragonball GT is still a project that was worked on and influenced by Akira Toriyama.
goku ss4 is stronger.He is stronger because he didnt need any help from bulma to go ss5.Also he has always exept for their first meeting surpassed vegeta.
No Goku does not turn SSJ 10 nor do any other characters, the max is SSJ 4 which only Goku and Vegeta reach.