It depends. It depends on what position you are applying for and when you got the relevant work experience. If, for example, the last time you did UNIX work was four years ago and you are applying for a UNIX position, then that reference from four years ago should be made available to your prospective employer. On the other hand, if your UNIX experience is current, then you should use current references.
If you can, make sure that you have a good idea what your references are going to say. Twenty years ago, one of my references with short-term memory issues managed to mix me up with another employee - I am pretty sure that's what happened - and said to my prospective employer that I required close supervision. That little prank cost me a job offer :( And no, I never found who was the smart ass who made that claim. Because nobody among my references owned up to it and my prospective employer wouldn't disclose whom it had talked to :) And needless to say, the frustrated recruiter who had come within one inch of getting her commission smacked me verbally and she smacked me good for being such a slob about collecting references :)