Is it possible to use a Verizon CDMA iPhone 4 with any other carrier or Pay-Per-Use carrier? This is probably seems like a FAQ but I haven't seen anything definitive as of today. 

I have a CDMA-Only Verizon iPhone 4 (not a 4S) that recently came off contract. I'm aware that it's CDMA only and GSM carriers like AT&amp;T can do nothing with it other than admire it as a thick iPod Touch. I'd like to explore options, mostly to lower my bill (I have no vendetta against Verizon, it's purely a cost issue).

So, network wise, are there any networks I can port to? I don't think Verizon has a MVNO in the Chicago area. Other than frequencies, is there anything I need to do to check compatibility with another network?

As far as unlocking, do I need to jailbreak my phone to change carriers? Or is this something Verizon will do (since it's off contract). 

If I need to jailbreak, is it a permanent jailbreak, or is it possible to change a couple settings and then I can revert back to tracking current iOS?

Are there any US carriers that can use the phone? Cricket seems to restrict it's network to Cricket-sold phones. Can Straight-Talk handle it? Virgin?
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.