What nonsense. There are tons of studies in other places that show much higher temps than in the Mediterranean. How can you even make your brain jump through hoops to convince yourself a +4F average would be localized? Romans had vineyards in the UK during this period.
Here is a study done in Lapland in Finland showing much higher temps there during this period.
Nature Climate Change (2012) doi:10.1038/nclimate1589
You'll have to internet archive it, it's been deleted by Nature. It's interesting that the wiki page on the Roman Warm Period was deleted in 2009 and now only has a couple paragraphs, with most saying there was nothing significant...lol. How about we try to grow grapes in Scotland?
Vikings described grapes and currants growing in abundance in Newfoundland during the Medieval warm period.