will my ADS-B IN work if I'm below 4000' (the MEA) between HSV and CHA where there is no radar coverage?
ADS-B does not require radar coverage to work. However, with the Skybeacon, which is 978-only, you will not see Mode C traffic when you're not in radar coverage, and you will not see 1090 traffic unless you're within range of an ADS-B tower.
Be glad you have 978 UAT...lack of flight tracking is a plus, IMO.
978 *can* still be tracked, just not all the sites do it. However, 978 does have an anonymous mode that will not transmit your tail number's particular Mode S code when you're squawking 1200, if your equipment supports it and your avionics shop set it up properly.
Also, when it comes time for your transponder check every two years, your check will be less expensive.
How so?
How would I get air-to-air traffic if it isn't painted by a radar? Does ADS-B out transmit without an interrogation from a radar?
Yes, ADS-B transmits constantly and does not require interrogation. Because the Skybeacon is only 978MHz, the only other planes that'll see you are the ones that either also have 978 or have dual band. If you're within range of the ground stations, they will rebroadcast your position on 1090 if you're within 15nm and 3500' vertically of a 1090-only aircraft. Likewise, it'll rebroadcast their position to you.
Its already been discussed in this thread, but also know:
most ADS-B IN receiver's software (e.g. ForeFlight and others) can display the location of active Ground Towers (GBT - Ground Based Transceiver) on your map. (I think you must have ADS-B OUT for this to work).
At least in ForeFlight, you can see the towers you're receiving on In (via Stratus, Sentry, Scout, or other devices) even if you don't have ADS-B Out.