I'm not a huge fan of FF4 myself (only due to having played it a billion times and sort of expanded from there) but it's a very good game to see the first time Final Fantasy really found its footing. It's a very seminal game, and for a console game from 1991 it's got a surprisingly deep/fleshed out plot with lots of themes that resonate well with people and became mainstays of the entire franchise. Love, sacrifice, honor, all that good stuff.
As far as party customization though, it has exactly 0. It's very much a story-first kind of game, and you have no control over which characters you have in your party, and you can't edit them aside from equipment. That also means that every fight is sort of designed around your party at all times though, so it's not all bad.