Given Ferrer is world number 4, is the current era noticably stronger than 2004-2007.
I have much respect for Ferrer, but would not expect to see him at 4.
He's momentarily #4 only due to an anomaly, namely Nadal's longer-than-expected absence. Prior to that, he was #4 only a few weeks back in 2008 when the current top 4 was only beginning to distinguish itself from the rest. You could always reverse the argument by asking whether that doesn't say something about the lackluster state of the top rankings prior to the current "big 4" solidifying themselves at the top. Ferrer being a regular world #5 is OTOH a fair ranking, but it's also clear that it's due to how much he plays (ESPN commentators noted that Ferrer in 2012 was the first non-"top 4" player to end the year with the most wins, for quite a while) and how consistent he is. There are clearly players ranked below him who are much more formidable opponents at any given day (Tsonga, Berdych, del Potro, even someone like Wavrinka) but that's how the ranking system works...