This is TICKET gacha only. Every so often they release a gacha that is in almost every respect a clone of the Premium gacha, but it also puts a ticket in your giftbox to redeem in a special gacha that only shows up when you possess a ticket. If you draw one time for 5 flower gems you get a ticket that can be redeemed for a random 4* girl. If you draw 10+1 for 50 flower gems you get a ticket to redeem for a random 5* girl.
Obviously, although it works out that it's the most cost efficient, you should only do the 5 gem draw if you don't care about 5* girls...but who would that be?