Growlanser IV is hailed as pretty much the highlight of the series. I don't know if that's accurate or not, since I've not had a chance to play it just yet, but yeah.
Looking at the screenshots and videos though, it really doesn't seem to play much different from the other titles, so I assume it's just the ability selection/story that make it herald that title. As far as other ones go, you can get English language versions of Growlanser II and III on PS2 in the Growlansers Generations 2-disc set, and Growlanser: Heritage of War (Growlanser V) as it's own release.
Otherwise, the first title was PS1, Japan only, and had a different PSP version come out a few years back, Japan only again. Growlanser VI is on the PS2, Japan only, and is currently the last title in the series.