UPDATE : I have been contacted by my retailer SCAN (by phone) to say replication of this overheating problem was successful, not only this but when they went on to test replacement CPUs using AIDAx64 and Intel retail HS/F they said they discovered a bad batch of three overheating 4790ks before the fourth CPU proved to work within spec under the same conditions.
Here is their screeny showing a CPU operating within spec at 4.4 turbo, running AIDAx64 under the retail Intel HS/F.
EDIT further discussion revealed this was made using a Zalman CNPS9700, not the Intel retail Cooler FYI.
So these CPUs will be returned and I hope they will stimulate Intel to consider its manufacturing process and package design. I hope this will be useful information for others who have thermal issues. I would suggest you could test using AIDAx64 and the Intel HS/F as a standard to compare like for like and if your CPU causes thermal throttling under those conditions you should probably RMA it and get one which doesn't.