I have the third and fourth editions. I think the forward to the fourth addition says there have been something like 150 hallmarks added? Which considering how many hallmarks are in the book total I don’t think is a ton. I would say try to get either the third or fourth addition and between the two it probably doesn’t make a ton of difference. @Chaparral compares and contrasts the two additions on this thread, which you might also find helpful for other book recommendations: