How do I use my iPhone 4 in Europe?

Contact your phone carrier. They'll give you all your answers.

I not to sure about this, but I think they can unlock your phone for international use, but you would have to use a local sim in Europe.

Call them back.

Verizon iphone 4's do not have a SIM. They are not world phones. They will work roaming in a few countries that still have CDMA networks, but you must ask Verizon to enable this capability. You are better off getting a Verizon iPhone 5, which is unlocked for GSM networks outside of the US, so you can just use a local SIM.

What about iPhone 4 from AT&T?

And what about iPhone 5s from AT&T

Are they unlocked or GSM ready ? If so then just getting the SIM will be all I need to do ????

iPhone 4 and 5s are GSM phones. If they are from AT&T, they are LOCKED to AT&T and cannot be used with any other SIM. You will need to contact AT&T and see if you qualify for unlocking.