FWIW i have a local branch with scintilla 4, looking good
Since Scintilla 5 with API changes is likely the next release, it is probably best for Geany itself to skip version 4 and go straight to version 5 since (IIUC) there will be no changes of dependencies or required compiler versions between those two. And any changes you have made for version 4 will hopefully be useful for Scintilla version 5.