I know that a 3rd weapon slot is a REALLY good idea but is 4 even necessary?
And would a bomb as my 3rd weapon work well if there were no "bomb enhancing" equipment? My 3rd wep if it wasn't a bomb would be an Argent Peacemaker and Divine Avenger, depending on the occasion.
I use all four spots every single run. I use all 4 because it allows me to cover all of the damage options, as well as have some cc, and a gun for the switches. My standard setup is either 2 swords 1 gun 1 bomb or 3 swords 1 bomb depending on if I can rule out a damage type.
I usually run DA/Faust/final flourish or combuster/da/faust if I'm going 3 swords, shivermist is almost always my bomb...saved my life so many times and sets up tricky kills like picking healers out of crowds.
It's not necessary but, it helps quite a bit. I use 4 weapons slots with Silent Nightblade, Glacius, Firo Driver/Blackhawk, and Gravaton/Freezing Atonizer occupying them. The reason for this is that even if you have 3 slots with one bomb one sword and one gun (or another combo), you still have the chance of running into a mob that is strong to all of your weapons. With my Nightblade and Glacius (Shadow and elemental damage) there isn't a mob that I can't do critical damage (large brown numbers) to. Hope this helps.