The "four friends" are Ted Williams, Bobby Doerr, Dominic DiMaggio and Johnny Pesky. They played together from 1943 to 1951, with time out to serve in WWII. They formed the nucleus of many great Red Sox teams, including the 1946 American League Champions. Williams and Doerr are in the Hall of Fame and, along with Pesky, have had their numbers retired by the Red Sox. While I'm admittedly biased, I submit that you will never find a greater foursome of TEAMMATES (not just players, but true teammates) on any other team.
(Though sadly, only 3 of the 4 make the 2016 Boston Red Sox All-Time All-Star team.)