My understanding is that the four year period gives anybody with an interest in the vehicle time to claim it. If after four years there are no claims, the bond expires and you can have the title re-issued as a clear title in your name, but I'm having a really hard time finding anyone that knows how to walk it through the tag office...I've bonded a few years ago but didn't hang on to any of them long enough for it to have the issue you're dealing with. Although I have never heard of one expiring.
When you read the bond details, you can get really screwed if someone comes forward with an original title and says its their vehicle. The bond company pays any financial "damages" to the claimant and then comes after you to get reimbursed.
I bonded a relatively inexpensive motorcycle that wouldn't have been a great financial hit if the bond went south, but I've been looking at bonding a fairly expensive classic car, but don't want to be on the hook if someone were to come out of the woodwork in ten years...
When you sold your bonded vehicle, did it transfer as a clear title or did it still have "bonded" imprinted on it for the new owner?