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.
Theres a program called DriveLED (not free), that monitors temps and does health checks:
http://www.oo-software.com/en/products/oodriveled/
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