P van Rijckevorsel writes
However I expect it will be a lot harder to find a set of four diploid
species. This almost surely will require a geographical component.

Stewart Robert Hinsley schreef
Perhaps in Azolla

Azolla caroliniana
Azolla filiculoides
Azolla microphylla
Azolla mexicana

http://www.botany2002.org/section11/abstracts/4.shtml
http://www.jaknouse.athens.oh.us/ferns/g_azol.html

It's not clear whether Aa. microphylla and mexicana represent one or two
hybridisation events. (Or even zero, depending on whether you accept the
position given in the first link,)

+ + +
I am not much on ferns, but I don't see anything on hybrids in these links?

In addition, with today's tradition of lumping most taxonomists would say
"four populations of plants that are interbreeding? Must be one
species"
PvR

========
Not always; at least I don't think anyone has proposed sinking the white
oak syngameon into a single species (Quercus robur?). More particularly, Q.
robur and Q. petraea are still recognised, as are Betula pendula and Betula
pubescens, even tho' both pairs of species hybridise extensively.
Similarly in Salix.
--
Stewart Robert Hinsley

+ + +
I know next to nothing about Smilax, but the taxonomy of Betula is only just
short of a regular nightmare.
PvR