It appears 4,000 was always the intended maximum mintage.
With the usual caveat of 1,916 in "this particular presentation".
They now, as you spotted, have increased that to 4,000 in that presentation, with a maximum mintage of 6,000 now (see specification tab on RM site).
You really shouldn't be surprised at anything the Royal Mint do now. Cash is king to them.
Collectors are an unfortunate inconvenience to them, always moaning about sky high premiums, mintages, milk spots, trying to cash in their 100-for100, etc.
They continue to hold the cocky view that collectors will continue to put up with everything, anyway, and keep buying.
And in the majority they are right. sadly.
You notice, also, that even after increasing the mintage on the Somme there is no consideration of a small percentage from each sale going to the Royal British Legion, for veterans, for example?
No, once they get your money in their grubby fists they won't let it go.