I don't have a Glowlight 4 or 4 Plus, so can't comment on those devices. I will say that I went from a Simple Touch (6") to a Glowlight 3 (6") to a Glowlight Plus (7.8"). I love the larger screen. Doubt I could ever go back to a smaller eReader.
From what I've read of user experiences on this forum, the GL4 Plus is a big improvement on the older GL Plus. If you can afford the splurge, go for the bigger screen.
