Ben,
WOW! What a story! After all of that you certainly deserve to be pain free four years post!
Don't fret too terribly much about a long term ProDisc-L. Per my research, if you can make it complication free for the first few years, you may be good long term (20 years anyway).
As you are in your early/mid-twenties, I have shoe laces that are older than you, but
here is a link to a study by Dr Zigler at Texas Back Institute that references a study began in 1990 regarding the ProDisc-L (I) by Dr Marnay in France. The long term outcomes were very good.
Good luck, Jeff