I am someone who travels internationally and is from the US, I purchased the unlocked international quad-band(GSM) version of the Samsung Ativ S more than a year ago. I have used it on the US carrier systems (before it was available from the carriers) as well as overseas. It sometimes shows 3G, and sometimes shows H+ (HSPA+ that some carriers sell as 4G) It does not however work with LTE systems. This should be a moot point in the US as Verizon is the only carrier I'm aware of with LTE service in the US and they are the only cellular service to use CDMA instead of GSM technology.
To summarize:
The Verizon version works nowhere else, but according to Verizon can use their LTE 4G.
The GSM version should work anywhere (except Verizon), and can use HSPA+ which is what the GSM carriers are selling as 4G anyway.