Comparing GDP contractions between Russia and US
In Rogoff's article (at least current version) the first sentence you quote links to IMF's World Economic Outlook Database. 

My guess is that when he is referring to output he means Gross domestic product, constant prices. Looking up the numbers in the database we have 63038.4 billions of 2011 rubles in 2014 and 61097.5 in 2016 which translates to 3.1% decline (negative growth). 

If I use per capita numbers than that also gives 3.1% decline which I guess is "comparable" to 4% decline.