If you breast feed 4 or 5 years straight, will it cause damage to your body?

No. Breastfeeding does demand a lot of your body (after all you have to share your nutrition with someone else), but as long as you're eating healthy foods and enough of them you should be fine. Just to make sure you're not robbing your body of something it needs you should take a multivitamin and some extra calcium. (your kids thank you for the breastmilk.).
Yes. The ideal interval between children has been suggested as 2 to 2 1/2 years to allow mother to replenish what she provided to her offspring. In case she has to continue breastfeeding longer than usual 12 months, she certainly needs to pay more heed in taking well balanced diet with adequate vitamins and minerals supplement to minimize any harm to her own body.