P van Rijckevorsel writes
It is conceivable that species A and species B hybridize at the
southern-most point of their range and that the result becomes species C. If
beside that species A and species B hybridize at the northern-most point of
their range the result may be different enough to become species D. I would
not know of a real-life example, but it is conceivable.

Stewart Robert Hinsley schreef in berichtnieuws
...
Found one (not geographical)

Species A : Cardamine amara
Species B : Cardamine rivularis
Species C : Cardamine insueta
Species D : Cardamine schulzii

C. amara and rivularis are diploids. C. insueta is a sexually
reproducing permanent odd polyploid (triploid, RRA), similar to Rosa
canina. C. schulzii is a hexaploid (RRRAAA).

Source: Verne Grant, Plant Speciation, 2nd edn, pp. 409-410.
--
Stewart Robert Hinsley

+ + +
Great!
Polyploids have their own mechanisms of sexual isolation come.

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