4x5 makes logical sense - which is something you don't associate with the States (!) - but 5x4 is more linguistic and pleasing somehow.
They'll catch up sooner or later. Either way, too much expense for me, so sometimes I say "4x5" out of spite and jealousy.
It's a curious fact that customary units in the USA are actually defined within the metric system. For example the length of the US yard is exactly 3600/3937 metres and the pound is exactly 0.453 592 4277 Kilograms. The change happened in the Mendenhall Order of 1893. An inch may be a convenient unit but you have to ask the metric system to be sure how long it is!
Yes, the legal definition of US customary units is tied to the metric system.
However, in 1959 the definitions were changed slightly, so the inch is now defined as exactly 2.54 cm, which is a few ppm different from the older definition you quote.
An interesting side effect was the need for yet another unit, the "US survey foot", based on the old definition of the inch, so that we would not need to re-define every land ownership document in the nation.
I refer to 1959 as the year that the US went metric, but did not tell anyone.
H L Mencken once said that when one-third of the people who speak a language from early childhood say "goods train" and two-thirds say "freight train", then "freight train" is standard and "goods train" is dialect.
(Incidentally, the first edition of the Oxford dictionary defined "freight train" as US for "goods train", but neglected to define "goods train".)