Sounds like some govn't guy got involved with tire ratings. The factory Maxxis site says the Bighorn 2.0 is a 6 ply tire. But it says 4PR on the tire (27 x 11.00 NHS 4PR). But wait, there's more...this is the lingo:
For example; A 4-PR rating does not mean a tyre has 4 layers of body ply layers, rather the 4PR rating indicates that the tyre retains the same strength as having 6 of the older style body ply layer
For example; A 4-PR rating does not mean a tyre has 4 layers of body ply layers, rather the 4PR rating indicates that the tyre retains the same strength as having 6 of the older style body ply layer