How to create a stream for matching

So I’m trying to create a simple game to learn some basics and I’m hoping people are awake to help me. I’m creating a simple Guitar Hero type game using arrows the player will match on screen with keyboard. Right now I have one arrow that spawns input is checked and then another is spawned and first is destroyed. My end hope is to have 4 on screen at one time the player goes through all 4 and another set spawn. Any help or suggestions would be appreciated as I’m not sure how to move on.