That’s an excellent question, and the answer is really quite simple.
See the pic below. Notice that the items all have a yellow background and when you hover over an item, it will say MAGICAL SET.
Underneath that, you see values for 2 set and 4 set.
Below is an example of a 4 set.
If you attach all items of a certain type (in this instance it’s the NECRO type) then you will get the whole 4 set bonus. Which as you can see above, will give you +15% to your attack.
But did you know you can also mix sets?
Below is an example of mixing a NECRO set and a GOLDEN SUN set.
Doing this will give you the 2 set bonus from both. So in this instance, you will get +5% to your attack from the NECRO AND +5% defence from the GOLDEN SUN items.
These are the only combinations that work.
If you have 3 of one set and 1 of the other set then you will still only get the 2 set bonus from the set where you own 3 items.
It is also possible to mix set and non-set as you see below, but as you imagine, you will still only get the 2 set bonus from this.
And as added loveliness in the latest version, we added VFX to the sets.
So now your character look will change even more as you equip sets.