4 years of heavy use and sure the HD might need replacing, but thats a £30 replacement. And yes you're right, if you're doing a big task it might better to upgrade to a new machine but in most cases it'll be fine even for photoshop.
Sorry I have a bit of a gripe with machines just being chucked out cos they're old*er*, we do it a lot at work and we throw out a lot of excellent PCs as some of you have received. My main machines are running fine and their 4+ years old.