Hmm,
@Bounty just pointed out to me that Blackbeard may actually have the 4th RP? His dialogue about Kaido "also having one" seems to imply he does as well, unless Aokiji or another Blackbeard pirate somehow also stole BMs RP.
Considering his history with Whitebeard, it does make sense that he might have it, considering Fishman Island was WB territory.
But I'm curious if that's the case, because then it must imply Blackbeard is the final major villain BEFORE Laughtale. I.e. he must he defeated to get the 4th RP and thus the way to Laughtale.
No idea what Shanks' motives are otherwise, but it almost seems like that one doesn't even need RPs to get there.