If I unlock my Verizon Iphone 4, will I still be able to have Verizon service?

I want to use my iphone with a different company, but I don't want to have a lapse in cellphone service.

The iPhone 4 is a CDMA only phone and will on;y work with other CDMA carriers. It will not work with GSM carriers.

Yes, you can have other service too a lot more choice

Of course, once you unlock your cell phone you will be able to use it with any carrier including the original carrier. Keep in mind that if you want to unlock your Iphone use only factory unlock for the following reasons:
It will never be locked again
Factory unlocking increases the sale value of your iphone by 50-100 dollars

Also you should know that there are companies out there that provide factory unlocking services online such as: http://www.cellsunlock.com that I personally tried many times for my phones