I do not have either of these and I see quite a lot of support for them but, other than V4.2 being newer than V4, I have no idea how old either version is, and I think V5 is for another program and would not work well, if at all, in Poser.

My Poser stuff is so old, I bought some clothes for, and a few figures based on, V3 and M3 but never got around to buying the base figures themselves as my old computer started misbehaving before I could do much of anything.

V4.x product durability would play a part but, before I try to save up some money to start a V4.x collection, I would like to know if the primary reason these are supported so strongly is because of product age (accumulation over time) or if it is because people are thinking this is the last Vicky Poser will see for a while and so vendors prefer V4.x over previous figures because "this is it, folks" (rush to end game).