Disney does not discount or pay commissions on 1 or 2 day tickets to any of their wholesalers, so would assume no chance that you would see any deals on them.
I was on undercovertourist website. The 4 day + 1 extra day is essentially a 5 day ticket? Because right below it is the 5 day ticket option. It confused me because why would anyone buy 5 day when you can get the day free? Is there a catch or limitation?