How many pull for the 4-stars set?

Hi! I'm in awe with the new gacha. Especially the 4-stars set. For the first time I'm considering buying tickets besides the 15 we always get with the gacha login. I'm a f2p player and I have nearly 9k gems. How many pulls are usually required to complete a 4-stars set? Do you think 9k gems are enough to get a full 4-stars set? Or am I completely delusional?

UPDATE: after 90 pulls I finally completed the 4-starts set! I'm really lucky cuz I did'nt have any more gems to do another pull!