The way I read it, that should be what the version Tabsoft posted does.
I might be seeing something else in my mind, or does it just scroll the wrong direction?

Basically, this could be written simpler if it never had to be a real program.
You could ditch the line buffer and rotate routine, step the index where you read the lookup table,
check it’s bounds, and write 20 char to LCD at a time directly to LCD.

Usually you want to look at, or change the display data, so I’d normally prepare
all lines of text in buffers, and write all four lines from the line's start positions for every frame.
even if the text doesn’t change or move.
If you never write past the end of a line the text can’t wrap.