Very very interesting! Three powder metal alloy steels and one vintage plain carbon. For the alloy steels, the iron-carbon lattice (tempered martensite) is the matrix in which the alloy carbides are embedded. For the Ward, it is all matrix, all tempered martensite, and given its wear rate, probably not many if any carbides (Fe3C).
Pure guesswork interpretation: For all 4 blades, the carbon steel matrix wears away first. For the Ward iron, which is homogeneous carbon steel, this leads to even wear.* For the alloy steels, the loss of matrix exposes an increasingly ‘rocky’ surface dominated by alloy carbides. More rugged and rocky according to the total carbide presence, thus highest for M4. Eventually, the alloy carbides get sufficiently exposed by loss of matrix, that the shear of the wood fibres begins to spall them off.
*Wonder how Hitachi white steel would wear—it is also homogeneous tempered martensite, but with carbides due to higher C content.