The iPhone 4 from Sprint doesn’t have a slot for a SIM card
Sprint is a CDMA carrier. They don’t use SIM cards as the technology for their service is built into the phones.
Newer model iPhones from Sprint come with a SIM slot as they are compatible with both CDMA and GSM carriers.
To unlock the phone you will need to contact Sprint. As long as the phone finance is fully paid off and not reported stolen or lost, Sprint will be able to unlock the device.
Once you have unlocked the phone then you will be able to use this with any carrier that allows Bring Your own Device for Sprint phones. Lots of the smaller carriers run off the Sprint network so you will have lots of options
Below is a link to an article about how to unlock Sprint devices as well as a search for carriers who offer Bring Your Own Sprint device. (BYO Sprint phone)