Get colored zip ties at your local hardware or maybe Walmart.
Either pick 4 different colors or 2 colors.
Place one around their leg just above foot and clip off excess with toenail clippers or diagonal pliers.
Endless combinations can be created to differentiate the chicks.
Billy is blue
Rhoda is red
Gilbert is green
Yolanda is yellow....
Or blue left leg is billy, blue right leg is burt
Red left leg is rhoda and red right leg is rhonda....
If you only have 1 color then one can have one on left leg, another can have two on left leg
The others can have either one or two on their right leg.....
Anyway, I 'm sure you can see the options.... the nice thing about zip ties is that they are cheap and can easily be clipped off and replaced as they grow.
Just remember to make a little chart to keep track of who is who so you can track progress, weight, feathering, ect....whatever features are important to you for future breeding purposes.