Here we see the pteroid bone which is the thumb but here it is not numbered "one" as it should be. This is because the current (incorrect) opinion is that it is not a thumb. When it is correctly identified as the thumb , then the picture is simple and complete. The five fingers (including the thumb as number one) are present in the pterosaur wing. And the little finger (the pinky finger) is the fifth finger (not the fourth).
For more info on the pteroid bone check here:
"The pteroid bone is a rod-like element found only in pterosaurs, the flying reptiles of the Mesozoic. It articulated at the wrist, and supported a membranous forewing in front of the inner part of the wing spar. The function of this bone, particularly its orientation, has been much debated. It is widely believed that it pointed towards the body, and that the forewing was relatively narrow. An alternative hypothesis states that it was directed forwards during flight, resulting in a much broader forewing that acted as a leading edge flap. We tested scale models in a wind tunnel to determine the aerodynamic consequences of these conflicting hypotheses, and found that performance is greatly improved if the pteroid is directed forwards: the liftdrag ratios are superior and the maximum lift is exceptionally high in comparison with conventional aerofoils. This high lift capability may have enabled even the largest pterosaurs to take off and land without difficulty."
"The interpretation by Goldfuss (1831) of the pteroid as the first digit, or thumb, and thus the wing-finger as the fifth digit, sparked off a protracted debate"
"The nature of the pteroid, a rod-like bone projecting from the carpus in pterosaurs, has long been disputed. Three lines of evidence, morphological, developmental and histological, indicate that the pteroid is a true bone, rather than ossified cartilage. The origin of the pteroid is unclear: it may be a modified carpal, the first metacarpal, or a neomorph. " [Or a thumb].
"A recent article by Wilkinson and colleagues (2006) on the
function of the pteroid bone of pterosaurs started with a statement that the pteroid has
long been controversial. Perhaps, but there have been two separate controversies widely
separated in time. The first controversy was but a part of the larger controversy about the
homology of pterosaurian fingers. Cuvier (1821– 1824) interpreted the manus as consisting of the small Digits I–III plus the hyperelongate Digit IV that supported the wing, whereas Goldfuss (1831) suggested that the pteroid represented a vestigial Digit I, the small fingers Digits II–V, and the wing-finger Digit V.
Although Owen (1869) sided with Cuvier, most authors (Wagner, 1837; Fraas,
1878; Marsh, 1882; Zittel, 1882; Williston, 1903) sided with Goldfuss, until Williston (1904,
1911) and Plieninger (1906) presented convincing arguments that the phalangeal formula of
pterosaurs, 2-3-4-4-x, is essentially un-changed from their non-volant ancestors except for
the possible loss of an ungual on the wingfinger. That view has been accepted by almost all
authors ever since (e.g., Romer, 1956; Kuhn, 1967; Wellnhofer, 1978, 1991a), although Unwin and
colleagues (1996) resurrected the Goldfussian view as a viable alternative to the
Cuvierian without actually supporting it."