Fighters make things easier but really you don't need more than 2. You can't realistically split endgame gear among more than 2 Fighters anyway.
Generally it's best to take at least 1 Fighter as your main physical damage dealer and so that you don't have a party full of squishy characters that die fast.
Thieves make things harder... If you want a real challenge play 4 Thieves.
All the other classes, it's not that hard to do a 4man party of the same class just for fun.
In my opinion Thief is the weakest class as they don't deal as much damage as Fighters and Black Belts, and they also die a lot faster. The Dodge stat sucks in this game and this class doesn't dodge as much as it should.
Red Mages are pretty strong for half of the game but they kinda taper off in power as they cannot use the strongest White and Black spells later in the game.
They can use some nice swords but of course Fighters are stronger physical damage dealers.
Black Belts seem kinda whack for most of the game because they can't use any strong weapons, and most of the Nunchucks are weak. After getting advanced job though they can dish out a lot more damage with their bare fists which hits multiple times quite frequently.
Multiple hits is godlike in this game and is often stronger than critical hits. Sometimes you can even get critical hits on your multiple hits which is beyond godlike.
It's always best to take a White Mage because Heals are pretty important in this game (items are weak and don't heal that much), but you could get by with a Red Mage if you take all the important healing spells on him.
Later on Healing Staff is godlike and you don't even have to equip it to use it. White Mages can also do decent physical damage with the endgame Hammers.
Same class parties are fun but in the end you will be struggling to divide up the gear between them. Unless of course you make 4 Black Belts, because then maybe you don't need to use any weapons at all.
In my opinion a balanced party would be 2 Fighters 1 White Mage 1 Black Mage. That way you can make use of all the best endgame gear and the endgame spells as well. You could switch out 1 Fighter for a Thief or a Black Belt just for variety, but overall the Fighter is much stronger than both of those.