Does this offer also apply for the newest cards?
Yes it works on the new cards as well, which is quite interesting to do once you cleared out the more expensive cards, since newer cards don't get the preview discounts.
Cashback is one of the best TGIF offers around, since you get to pick all four cards (no additional cost for re-picking), and discounted cards count towards the four picks. If you get your first three cards at a 20% discount each, and a free card at no discount, you'll effectively have a 33% discount on those four cards put together. Milage may vary a little depending on what cards you pick, ofcourse, there's some cards sold at higher prices, which are great to put as 4th card, but there's also the old Virtuagirl Classic cards (if you have access to them and an interest in them) which are very cheap to put as first 3 cards.