I only have 2 people in my family who play Xbox. I was wondering, if we get the pack, what happens to the remaining 2 years of Xbox Live?
They will go to waste as they are unused.
The pack is for four memberships but as it's currently less than the price of two it's still a good deal:
The Xbox LIVE Gold Family Pack brings your family four 12-month Gold Memberships for less than the price of two. Also enjoy Family-Pack exclusive features.
The other 2 unused memberships will expire at the same time as the other ones whether they are used for an account or not. For instance, you can't buy the family pack and get 4 years of membership out of it one after another, or in your case, get 2 years out of it. Once the membership starts, all 4 accounts expire at the same time.