I looked at Rio's website.....and sure enough......it states, "a full line size heavier than the industry standard".
I looked at the date on the calendar....it's not April 1.
I don't get it, either.
edit: I couldn't leave it alone. So I cheked to see what the actual weight of the lines were, compared to the AFTMA standards.
It looks like all of the line weights are right at the bottom edge of the next weight up. So the new Rio Grand 4wt line weighs in at 'just' being a 5wt line.
I still don't get it.