A wheel has come off here?
Aren't you the one who is supposed to answer such questions, rather than
pose them?

Not sure this question has a real answer: the "Basal tricolpates" are
something of a mess in this respect with Menispermaceae having a 3-merous
perianth, as well as Berberidaceae, etc.

I will play it safe and quote from Judd & al (Plant Systematics, a
phylogenetic approach, 2nd ed, 2002):
"Core Eudicots:
[...] These plants have usually 4-5 merous flowers with the perianth
differentiated into a calyx and a corolla; placentation is ususally axile"
p307-308

PvR

=========
Stewart Robert Hinsley schreef
Of which group of angiosperms, is the trait of a 4- or 5-merous
perianth, of two morphologically distinct whorls, an apomorphy?
--
Stewart Robert Hinsley