I noticed I can only add 3 mod slots to some of my purples

[Heisenberg]

It shows space for an additional slot but when I click it open in Salvage Matrix it says "No options available". Why is this?

[Shima]

Do you have it equipped in a loadout somewhere? Should be able to add 4 to anything provided you have the resources.

[Niquara]

sometimes weapons come with slots already filled...

May be part of the reason the weapon was purple in the first place....

[Natjur]

Adding a 4th mod on a purple weapon costs a TON. Most likely you don't have the cash or your currently using it (on any of your layouts)

[Audioholic]

Because adding an extra 2 barrels to a sawn-off shotty making it do 30k damage would be broken.