Yarn sizes?

4ply yarn whats that in america sizes i have a american sock that calls for sock yarn i thought it was the same thing but it might not be help before i cast on?

I’m not an expert on yarn sizes but I’ve found this chart on Lion Brand’s website to be useful in the past:
http://cache.lionbrand.com/cgi-bin/yarnInfo.cgi?yarnByClass=1
(the left-hand column)
According to this your 4ply is a good weight for sock yarn.