More intuitive menu design, the ability to switch out party members, less time between action.
Everything is just tightened up from III, menu layouts, response time, etc.
Audio/visuals are improved over III as well.
Basically everything is upped but the story is lackluster, III's story wasn't incredible either so I'd go with the game that plays better.
You're out of your mind if you think field controls do not matter. IV has analog support as well as better camera controls (although both are not that great).