An era if there is such lasts about four years.
Player at peak.
Thereafter decline & winning post peak.
Boris, Ivan, Mats & Stefan 85-92
Guga, Leyton, Marat 97-02
Rafa 08, 10, ?
Nole 11, ?
not sure how you came up with 4 years but OK. Rafa was dominant in 2010 only when he won 3 slams. Edberg, who I truly love was a great champ but never really dominant. I agree about Borg and Ledl they were dominant at their time and yes they did create their own era. Sure Sampras and Fed are their too. Nole is in the process of creating his era. how far he'll get. don't know. we'll see.
as much as I love Guga, Safin and Hewitt I don't think they had their era. Although, they were able to win slams and being #1 they weren't too consistent. It took Safin what like 5 years from his first USO win to the AO win. Hewitt again to me was just a transitional champ in a way before Roger showed up.
so if I really have to define this 'era thing' I'll go something like this (since I started watching tennis)