Is it legal for my employer to suspend me for being 4 minutes late? What is the time allowed to clock in with out being punished?

Yes, it is. This is not an area addressed by law. The employer does not have to provide a grace period if they choose not too. I'm not saying I don't think it may be petty, but it isn't illegal.

In fact, it would be legal, albeit petty, to suspend you for being 4 seconds late.