The hard limit for the legacy Windows console was 9999. I'm surprised to see that, in the new console's dialog box, MS have still set a limit of 4 figures (therefore 9999) for the maximum screen buffer size. Is this a technical limitation or just the UI refusing to let you go beyond this? If it's not a technical limitation, how can I increase it beyond 9999?
The technical limitation of backscroll buffer in Windows (not only Win10) is not a 9999 but 32766.
You can't change it via console window properties but you may write small program which calls
SetConsoleScreenBufferSize and set it to desired value.
Or just use alternative terminals like ConEmu.