All Verizon phones will only get 4g(HSPA+) on AT&T.
Posted via the Android Central App
This is assuming the Verizon phone is even equipped with HSPA+, which is a GSM technology. Verizon uses CDMA, so their phones support 1XRTT, CDMA, and other CDMA technologies.
SOME of their phones are built with a multi-radio chipset that includes support for GSM/GPRS/EDGE/HSPA/HSPA+, but by no means all of them, and I'd venture to say they are actually the minority.
LTE is LTE. However, different carriers use different frequencies and their phones are often built to only support the frequencies they own. So while a Verizon LTE phone *may* happen to support frequencies that Verizon doesn't use for LTE, it's not necessarily the case.
The utopia of taking your phone from carrier to carrier in the US is still years away, unfortunately, and Verizon is particularly effective at preventing it, whether it be bringing non-Verizon phones to their network, or porting Verizon phones off their network.