What does q4/q4 mean?

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:

What is YoY & how does it work?

YOY also differs from the term sequential, which measures one quarter or month to the previous one and allows investors to see linear growth. For instance, the number of cell phones a tech company sold in the fourth quarter compared with the third quarter or the number of seats an airline filled in January compared with December.

What is a yoy comparison?

YOY comparisons are a popular and effective way to evaluate the financial performance of a company. Investors seeking to gauge a company’s financial performance use YOY reporting. Year-over-year growth compares a company's recent financial performance with its numbers for the same month one year earlier.

What is the YoY growth estimate for 2018q4?

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. Please see update above.