In modern football, especially if you're of the more cynical disposition, contract extensions for managers really don't mean too much. They really aren't guarantees of anything other than a paycheck and/or a severance package.
They're a bit like preseason. Or maybe the Community Shield. Not quite meaningless, and better and more confidence-inspiring if we achieve positive results (i.e win or extend), but hardly telling indicators of the future. We only have to look back to the long-term extension Mourinho signed with Real Madrid in 2012, only to be on his way out of the club a year later. Or the multi-year full-time contracts Chelsea have given to interim managers Avram Grant and Roberto Di Matteo, neither of whom lasted longer than 6 months after signing.
Where I think it does carry a bit of significance is that it is the first managerial contract extension of the Abramovich era. Jose never got a new contract during his first stint with the club, and we haven't held on to a manager longer than two seasons ever since. Mourinho is about to start his third season and he's now signed the first-ever contract extension authorized by Abramovich, the occasion of which even prompted the first-ever words in the media from the mysterious and powerful Marina Granovskaia.
"We are very happy that Jose has committed four more years to the club. Since his arrival two years ago he has carefully developed the playing squad and brought trophies to Stamford Bridge. We look forward to the next four years and the continued success of the team."
-Marina Granovskaia, source: Chelsea FC
Presumably, as the long-brewing rumors indicated, key staff Rui Faria and Silvino Louro received new contracts as well. Either way, Mourinho is very happy with the way things are turning out.
"If the club is happy, I am happy. I think this is a normal thing for me to sign a new contract. It is important we have this continuity and I hope we can enjoy more success in the future - for the fans, the players and the club."
"I said when I returned here two years ago that I have a special feeling for this club and nothing changed. It is the club closest to my heart and I am very happy to know I will be staying here for a long time."
-Jose Mourinho; source: Chelsea FC
Mourinho is three games shy of 300 in charge of Chelsea. He's six games behind Tommy Docherty for fifth all-time in Chelsea games managed. He's won more games than all but one manager (and Calderhead had nearly 1000 matches to get to 384 wins) and he's won more trophies than anyone else in our history. He may have his critics, he may be a pain to deal with sometimes, but we're unbelievably lucky to have him and to have had him for 5+ seasons in total now. Hopefully there are at least that many more to come, with just as many wins and trophies as before.