Yes you are right about the Big 4 moniker, in 08-10 period they were the top 4 and above the tour. Djoker and Murray were the minor partners in this league while Federer and Nadal were a cut above them, this changed rapidly in 2011 when Djokovic burst like a volcano and now despite the Bi4 moniker still intact and it would continue to be until 2017 but now Novak was also a major partner along with Fedal, this ensured that Murray remained out of place. If Hewitt and Safin peaked in 04-06 with Roddick then even Federer-Roddick-Hewitt-Safin could have been a big 4 but they Federer would be the major partner here. Same is the case with Big 3 in the Big 4 narrative and thats why the Murray comparisons does not look very sound.
Safin cannot be overrated, he has beaten Sampras, Federer, Agassi, Djokovic all in slams, he has even beaten Agassi at the French Open. He is the only legit guy out there to bet on when in form, not even Wawrinka...Lets not forget Stan is 0-23 vs Federer outside clay

Thats basically David Ferrer territory. I dont see Wawrinka beat peak Nadal or Pete either. Safin is not at all overrated IMO.