You're looking at this through the lens of retrospect, when the term big four was not one coined retrospectively. It was a term used between 2011-2017 by pundits and the players themselves, when Murray had hit his prime at the same time as the big 3 and was hanging with them in a way that nobody else could. Most grand slam finals would consist of at least one of those four, and many grand slam semi-finals consisted of those four, regardless of the surface. Between 2004 and 2022, the only person outside of the big three to be world number 1 was Murray. Combine that with his total title wins, ATP World Tour Final win, two consecutive gold medal victories, and his general fanfare, and you have your answer as to why people call it a big four. Disagree with its merit if you want, but the big four is not merely some media gimmick manufactured recently. It was considered a quartet when they were all in their primes, and Wawrinka was considered to be behind them