Is it possible to use a Verizon CDMA iPhone 4 with any other carrier or Pay-Per-Use carrier?

Not likely unless Verizon agrees to release that phone for another carrier. Have a look at this guide on identifying iPhone models:

• Identifying iPhone models

The iPhone 4 you have is model A1349 (that should be printed on the back of the device), and if so, it is a pure CDMA phone so you cannot connect it to a GSM carrier due to differing radio technology.

Even if you were to jailbreak it or get Verizon to unlock it, it's only going to work with another CDMA carrier.

So in the USA - you need to find a carrier with coverage in CDMA EV-DO Rev. A (800, 1900 MHz) and that's primarily Sprint and Verizon. Since there is no SIM card in this device, you might not need to get it unlocked, but that will be a dialog between you and your potential carrier if they can provide coverage to it at the same time as Verizon may have it registered as "their locked device"

Due to bilateral roaming agreements - each CDMA tower in the US can and does serve multiple carrier devices - so it's likely that Verizon could prevent you from using your device on a competing carrier until you fulfilled your contract or convinced them to release that device's status in the common database for cell phones.

There are well over 50 regional and national carriers providing CDMA coverage according to this wikipedia article so you might have considerable options in some regions of the US.

_{Nore some of the information is old due to consolidation. Your main bets are C Spire, Sprint and Verizon.}

As far as I can remember, Verizon uses CDMA networks (as opposed to everyone else on GSM). You can identify your phone model by looking at this Apple chart.

If the serial number of your iPhone 4 starts with A1349, then your phone is CDMA and no "big" network will accept it other than Verizon. But not all is really lost.

AT&T and T-Mobile (and Europe for the matter) use variants of GSM which is not compatible with CDMA.

So if your phone is CDMA, the answer to your question is: Only Verizon can use it (it doesn't have a SIM card as far as I can remember). (This is not 100% accurate).

Now, will Verizon unlock it? Possibly if it's off-contract but I'm not sure how they work. AT&T would do it.

Jailbreaking the phone won't (as far as I know) give you much, because the problem is not in the software, it's the hardware. The communication through CMDA uses different frequencies than those of GSM.

Now, the truth is, that there are other companies (depending where you live) that will use the Verizon Network (these are called tier-2 or tier-3 companies) and are usually local smaller companies that "rent" the big guys network (and pay them for that!).

These smaller services are usually more flexible because they are small and more efficient. You named Straight-Talk which is a good example because they lease Verizon's network and should have CDMA. You might want to talk to them to see if they can help you with this.

Just call H2O wireless, they will connect you in minutes, 80 minutes for $10.00 which is their minimum; it is a pay as you go service, however they have other plans as well, no jailbreaking or unlocking is necessary, happy talking!!!

I just called Straight Talk and they told me that I could not activate my Verizon iPhone 4S on their network.