Checking in with Delta @LHR,is 4 hrs before dept enough time?
 

also,is it worth saving some $$$ for Duty Free? Real deals and great selection?

"<i> Checking in with Delta @LHR,is 4 hrs before dept enough time? </i>"

I'm super conservative - but 4hours is waaaaaaay more than enough. 3 hours is plenty, 2.5 should be OK (But you should check Delta's website to see what they require)

"<i>also,is it worth saving some $$$ for Duty Free? Real deals and great selection?</i>"

Lots of Duty Free - very few bargains

You will be bored silly arriving so early. Agree with janisj..three hours is plenty.

Plenty of the goods in duty-free no longer have duty to be free of. Booze does and remains a bargain.
While we're at it, I am always puzzled to see shops inside the secure area of the airport selling suitcases. How can that work?

Once we were at Heathrow more than 3 hours before our flight on British Airways. We weren't allowed to check in until 3 hours before our flight. We had to sit around on uncomfortable benches until that time instead of waiting in a comfy lounge.