There no reason to this gen to be shorter then others one ,it's definitly possible to ps4 to outsell ps2 in europe and usa but it japan no but maybe other country like Vhina will replace the difference between ps4 and ps2 sale in Japan.
PS4 - over 100 millions let's say 120m
Xbox One - 70m
Wii U - 25m
Vita - 15m if it will not get Final Fantasy Kingdoms Heart and Monster Hunter 20m otherwise
3DS - 80m