I assume your current piggies are both female? How old are they?
The bonding process is exactly the same as normal.
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
You do need to be prepared that merging two pairs is not always easy particularly if all four piggies are of a similar age, as neither of the dominant piggies in each pair May be willing to give up their position as number 1.
Adding More Guinea Pigs Or Merging Pairs – What Works And What Not?
If you wish to add more piggies to your current pair, then doing so via dating at a rescue centre is safest as your piggies an choose their own new friends. I'm not sure how often skinnies come up though.
Skinnies are harder work to keep as they need to be kept warmer, eat and poops so much more.
A 4x4 should be big enough for four piggies if the bonding works.