Hey there RedSoxDoula,
Not a Yankees fan I guess?

I have not seen a client that had a previous 4th degree but I have had enough clients to know that just because one tore before does not mean they will again.
Some of the reasons someone might tear more besides bb positioning, perineal support, and slow or fast pushing, have to do with vaginal tissue health at the time of birth. For example infections during pregnancy, or even just one close to the birth (yeast, std’s or bv) can all cause the tissue to be more friable.
We had a client once who had multiple yeast infections and she tore pretty badly (not to mention nuc hand and lrg bb). She had a third degree that when into the anal tissue but not the anal muscle band, so technically, it was still a third degree. We decided to transport her for suturing with a surgeon, just to make sure it would all be perfect (although we do suture most things with confidence). This same client went on to birth again without any major tearing 2 years later. I would be aware of your vaginal health and be on top of any infections if I were worried about that aspect.
If you decided to interview midwives for a homebirth (there is a list of them, if you don't already know, at
www.massmidwives.org) you may want to have one of your questions be about their suturing skills and what they would do in different tearing scenarios.
Also, if you are having babies very close together this may effect the possibility of the scar tearing as well.
Best of luck, Paige