Been brining a 1/4 turkey breast 4 days in fridge with pomegranate juice, meyer lemons, kumquats, salt pepper balsamic vinegar - it's grey in parts - is it still OK to eat? Smells great but that may just be marinade?
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.