4yrs is quite old for a HDD. Like any drive it might well fail but unless you've noticed it making unusual behaviour (tell-tale signs) ie: clunking noises, or r/w errors, or if it goes slower maybe (even if its not fragmented). Its probably nothing to worry about.
No sense in worrying bout it unless theres a specific reason, has your drive been acting strange?
Oh, and you should go on Seagate's website and see if they have any programs that can monitor your drive, I know western digital have but not sure with the others.
Also since your drive is old the capacity is probably quite small so if you do have fears, I'm sure you could pick up a new drive that can easily backup all the files on that HDD. Quite cheaply if you only want a small/similar sized drive, 120gb is cheap cheap cheap these days
You can try to defrag and then do an error check, If windows can not fix the prblems, keep on trying 'till you think you need to reformat the drive.
You can also enable the S.M.A.R.T. monitoring from your motherboard bios as an added measure.
If you need to format the drive, try formating it 3 times or more with partition release to whole amount of your hard-drive (no partition). Then partition your drive accordingly on your final format. Then install your operating system if needed.
2 of my 8 year old drives are still running on two defferent computer running on Windows XP and application. One HD I cloned and running it with the new ones, Saving all my files with the new drive only. One system only has USB drive to backup the data.
I'm 80 percent sure i'tll still work for a year or two.
Signs could include....
Long read/write(it messes up and has to retry allot)
Clicking (click of death)
I sugest if you are worried that you get an external or internal(unplugged when not un use to keep it clean of bad stuff that may get into your computer...) for back-ups....let say every week....
i have a 5 year old maxtor(before those damn slimline death drives came out...) thats been running 24/7 for at least 2 of those years....drives are good now days....but as always...back-ups are your friend...