We're talking about 4 sets WITHIN a set – 12 reps – with an exercise; in this instance – an Incline DB Curl.
Do it this way and you get the MOST out of it. Why waste a good exercise?
Is your energy so boundless you can dribble it away? If you try to do 12 reps any other way, you'd be "swinging" the weight up after 6 reps.
Here's what I do using a Seated, Alternate Dumbell Curl as an example.
You start with this.
When you get strong in the movements, you go to curling the Dumbells together at the same time.
1) Four Alternate (they're the hardest)
2) Four "together" (because they're easier)
3) Four from the shoulder – "down" ("negative style") – then up. Singles.
Every 4 reps I go into easier applications so you continue to DICTATE to the weight.
At last, I developed this One Down – One Up thing.
It isn't WHAT you do – it's the WAY YOU DO IT. It is what you put into it.
Invariably a guy will ask me for more exercises. He isn't putting his "all" into the routine. He assumes he needs to do MORE exercises. Actually, what he needs to do is put MORE into what I've already given him.