I had the same tearing as you 2 years ago. I feel for anyone that has had to go through this; as the road to recovery is long and painful! theres not enough support in the medical field to help ladies that have suffered from these tears! Is hard as theres not enough info out there (nor studies that are conclusive!) to show the best mode for future deliveries after such a nasty tear.
Im having my 2nd bub in 3 and a half weeks by caeser, as with my tearing I have very painful scar tissue, incontinence issues and wasnt stitched property. In my case im going to have further recorrective surgery to help fix my bits (and hopefully get rid of scar tissue!)
Id suggest that if you have faecal incontinence to have an elective caeser as you dont want to further damamge those muscles! Lots of women that have had 3rd degree tears go on to have more bubs naturally without any further problems, with a good ob and a managed delivery I dont see why you cant go on to have a natural birth.
I didnt know before I conceived, that ob/gynos can test your bits after 3rd and fourth degree tears to see how well its all healed, and to check for faecal incontinence and deep scar tissue. They insert a small probe into back passage (sorry graphic deatails!) to see how well its all functioning etc. Generally its done when baby has finished breastfeeding due to hormones and your bits going back to a semi normal state.
Id go armed with all the info you have, but you have had a 2nd delivery with no probs so I think youll be fine!
best of luck!