There are actually a few factors to take into consideration when discussing this. 3-4 years ago this was a good philosophy, but I do not think it should be set in stone. This year alone our software load on our computers doubled, and in that load we begain running two sql servers on the local drives. Most of our computers were still in the last year of their duty cycle and most began failing because they speced for far less strain, unable to keep up.
Then there's the issue of knowing the software you are using. Because of the addition of the localised SQL server we have had to up our price point on the systems we spec because with the previous price point we could not afford equipment with enough power to operate well.
I would agree with the 4 years for a desk top and 3 for a lap. Mind you, buy a Vostro 400 and 4 days will be about right :smileymad