Brining Turkey - 4 Days in Fridge - Partially Grey

The color is likely due to other intensely colored ingredients in your marinade, notably balsamic and pomegranates. They can discolor other items so I wouldn't be too surprised if this happened to the skin and outer surface of the turkey. I doubt that it indicates a hazard.

4 days is longer than our typical guidance on food storage (which in turn comes from governmental food safety agencies) would say is strictly safe. 1-2 days is generally about the limit for uncooked poultry, but all the acid in your marinade should make a fairly unfriendly environment for microbes, so this is kind of a grey area.

Personally, I'd be okay with eating this so long as I wasn't serving it to the pregnant, the elderly, or anybody else with a compromised immune system.

You don't mention whether you intentionally let this marinade for the full 4 days, but if you did I'd advise you to back that off to 2 days at most next time. The acids present can also affect the texture of the meat, and will probably completely cover the flavor of the turkey itself. You'll have a more pleasant result and one that doesn't run into questionable food safety territory next time if you reduce the marinating time.