I am currently looking to purchase a used iphone4s solely for testing apps. I was wondering if there are any service requirements like "you must have a service plan or the device will not operate". I just want to sideload apps for debugging.
If an iPhone does not have a SIM card, it will work with WiFi.
However, if the iPhone is locked to a specific carrier, you can only activate it by plugging in a valid SIM card for that carrier. Once activated, the iPhone will be usable without a SIM card.
In the US (I am assuming you are in the US), most iPhones are sold as being locked to a carrier, and unlocked iPhones are rare. Outside the US, unlocked iPhones are more common.