In reply to
james and lex:
A 4 year old harness may be totally fine, or it may not. I've seen one harness which was dangerously worn after 1 climb: the climber had come off second best against a rough granite chimney!
For piece of mind, follow the manufacturers advice on lifetime. This varies between manufacturers, but as an example Petzl advise a maximum of 10 years from the date of purchase. This means that even if the harness looks totally fine, and has never been used, it is recommended to scrap it.
In addition, you should regularly check your harness for dangerous wear and tear. Check the webbing, load bearing stitching and the buckles. Belay loop and tie-in points take the most hammer, check these carefully. If you don't know how to check your gear, Petzl have some useful videos on their site which can give you some advice. You can also download the free BMC Care & Maintenance booklet here: https://www.thebmc.co.uk/Download.aspx?id=77
Dangerous chemicals, such as acids will damage your harness and may not cause visible damage. Keep your harness away from them - what did you have thrown in the boot of your car last week?
Lex, if your sewing is up to it, then resewing a non load bearing part such as a retaining loop should be OK. It won't damage the webbing. Bear in mind that any kind of home repair will invalidate any warranty though.