What is the difference between Q4/Q4 and Y/Y GDP growth? Per US media, the US economy grew 3.1% in 2018 Q4/Q4 and 2.9% in 2018 Y/Y. 2.9% seems the reference number in various databases (World Bank, Reuters, Bloomberg, etc) but what exactly does Q4/Q4 mean and which measures changes from Jan 1 2017 to  Dec 31 2017 compared to Jan 1 2018 to Dec 31 2018
If you could include the source(s) you were talking about that would be great. Without further info, my initial guess is that the Q4/Q4 means you compare GDP in 2018Q4 to GDP in 2017Q4. Y/Y means you compare GDP in the whole year of 2018 to that of 2017.

Upon further inspection, if you take a look at the US's real GDP, you'd get the following:


Growth in 2018Q4 compared to 2017Q4: 2.52%
Growth in 2018Q3 compared to 2017Q3: 3.13% [Link]
Growth in 2018 compared to 2017: 2.93% [Link]


So I think your Y/Y number is right... but the number for "2018Q4/Q4" might have been growth in Q3?



Update

It seems like the number reported was the initial estimate (which gives YoY growth of 2018Q4 to be 3.1% and 2018 to be 2.9%) in February 2019. The third estimate (which is what FRED uses) was released in December 2019 and yields YoY growth as calculated above.