The 4th characters are usually specialized.

As for your 3 initial characters definitely train one character with swords because the 2 best weapons in the main game are swords. You'll probably want to train each of your initial characters with different weapons so you can make use of as many weapons as possible. You should try to make all your characters decent at physical fighting so they won't be using magic all the time to attack enemies. As for magic it's recommended that you teach all of your main characters magic. It's up to you whether you want to make them stick with either White Magic or Black Magic or make them specialize in both. I personally prefer making them use both White and Black Magic. It may take longer to train them and make their magic more effective but your character won't be useless against high defense enemies that they're facing alone if your other characters get killed off. Also enemies can drain your MP and it's important to have someone else that can use magic if your main magic user loses all their MP.