Big Horse Shop.
My Suffolk wore the 4XL when he needed them. In terms of size, the diameter of the bottom of the boot is the same width as H&H magazine, you could wear them as a hat.
3xl don't exist (I have researched extensively).
In 4 xl you have 2 options: Big Horse Shop or Davis Draft bell boots. The Davis ones are much more robust but i expect more expensive also.
As O&L says I have worn them as a hat previously! My clydesdale was in m2m shoes and they just about fit him. If you just have hunter feet you may find them too big.