Scoobidoos! Argh, there was a craze in my school for at least a term and a half and it drove me up the wall; people would walk around with their bags drippings with (literally) hundreds of them, even in the sixth form!
Yours idea is MUCH cuter though - who knows? Maybe I can bring the craze back again! (Or not.)
Any ribbon will work. The ribbon here is grosgrain but satin ribbon or any other type will work. Your four different ribbons also do not have to the the same size (width). If you use different widths of ribbon, the bracelet will not be square but some other interesting shape! You could use string but it would look more like a 4-strand braided friendship bracelet.
Yours idea is MUCH cuter though - who knows? Maybe I can bring the craze back again! (Or not.)
Oh or could you use string???