FF4 is simple, a good game and overall a very good one to start with but i prefer FF9 because i'd say its better.

FF4 has abou 20-25 hours of gameplay and is easy to follow so i guess FF4 would be a very good one.

FFV would also be good to start of with but it might be a bit hard if the players new to the job system.

FF9 is easy enough and is possibly my favourite FF and i've always thought of it as a good fun game which can be played by anyone.