So when I see this:

REQUEST UPGRADE (for all travellers), requires 2000 upgrade miles or 1 confirmed N America upgrade.

Means what is says, that the flight is between 1520 and 2000 miles and you need to use your 4 x 500s for that one way flight per person.

You can buy extra 500 mile upgrades if you need from UA, or go to Coupon Connection and see if you can trade for a "confirmed N America upgrade/s" (usually termed 'HK49s' by those here) that generally only 1Ks can earn, but can transfer.