Four-star Card
Log in daily to get daily log-in rewards like Crystals, and Magic Tickets. Both of these currencies can be used to buy packs in the Store area on the main menu having a chance to get a 3, 4 or 5 star card.
I have used 5 Magic tickets so far and got two 4 star cards and three 3 star Cards so this is going to just be luck based.
You can also do events to earn Crystals and Magic Tickets by events and playing the game which are always changing so I can't exactly put them in this solution.
Good luck to all and have fun.