Selling iphone 4 from verizon?

If i sell my iphone 4 to a friend who has at&t, will they be able to use it since verizon doesn't use SIM cards?

Nope!

Sprint and Verizon Iphones are CDMA… Though they do have Global ready GSM radios for international use, Verizon and Sprint have blocked all access to ATT and Tmobile GSM services in their CDMA Iphones… There's no known hack since they put it in the main code…