There is no tier 4 clothing. The highest is 3, and they have 3 sub slots. You can randomly obtain new clothing from the shops everyday by buying them, or you can order them through Spyke by talking to people in the main hub of the game and ordering the clothing that they wear. As you level up higher and higher, you will start seeing tier 2 and tier 3 clothing appear in the shops. Keep in mind that it is a lot more expensive to buy clothing ordered through Spyke than it is from the shops, but since the shops update their stock randomly, you can get exactly what you want through Spyke.