Any clues as to why you think i is on the way out ? Also, is it that you perhaps feel you paid a premium price and expect longer ? I'd say 4 years is not too bad, given warranty periods of 1-3 years are common.
You might look online at Ebay, from a business seller, perhaps getting a year warranty, or worse, at a local store like CashGenerator / CashConvertor / CeX (I only say worse because prices are often higher and warranty is likely to be 60 -90 days).
It depends on how much or little you want to spend, as a new laptop may be 250 - 500 (recent threads included some with 6GB RAM for around 300-350, but brand names, and retail prices are very variable, and I'm far from convinced spending a lot more than 350 will get you a much more reliable machine).
If you spend around 350, and it lasted 4 years, do you think you'd be happier (I'm guessing you spent quite a bit more for the Vaio and if you sell it now, while still working, you may get half the cash towards a cheaper new laptop).
I'd look for Dell and Toshiba if buying second hand on Ebay. I've bought HP, Dell and others over the past year, generally spending 100 to 200 but in my case looking for Windows XP, so they were at least 5 year old machines (and most had keys missing, or similar minor problems... not a great problem for me as I tend to use laptops with second screens and USB keyboard and mouse, so I have consistent keyboard layouts on all my Windows and iMac machines).