I had some indication before the Arc was released that the P4 might be discontinued. I can't recall what it was without digging it up. Maybe a conversation with Leatherman customer service (who never know about discontinuation until it happens) and also maybe the P4 disappearing from online vendors briefly.
It would sorta make sense that it disappears, now that the Arc is basically the same MT for a $70 upcharge - though not everyone agrees with me on that one.