Wikipedia says the proliferation of these 4 and 5 stories of wood construction above 1 story of steel/masonry happened because of a 2009 revision to the International Building Code.
If that is true, then it would seem like one plus five, one plus four, 5 over 1, and 4 over 1 buildings would refer to how many wood floors the building has above the first floor.
https://en.wikipedia.org/wiki/5-over-1
I do not know how good the source is for Wikipedia, but I have only ever seen 5 and 6 story buildings, and never 7 or more stories with the first floor having masonry/steel and then wood for the rest.