It's all rather confusing. A few years ago, Abena renamed the X-Plus using a letter to indicate size and a number for absorbency. So a medium X-Plus became an M4.
Then, about 18 months ago (I think?), Abena officially changed their plastic-backed Abri-Form diapers to use cloth-like backsheets and introduced the Abena Premium range (aka Air Plus) which were almost identical, except the backsheet was breathable and there was a bit of elastic in the tapes.
After many complaints, Abena decided to unofficially continue production of the plastic-backed M4 and L4 in the US, and some European resellers also seem to have got hold of some.
There don't seem to be any UK sites you can order the plastic-backed Abenas from any more, but Save Express in Germany still sell them:
http://www.saveexpress.de/en/inkont...m-slip-weiss-15.25.03.1106-folie-14er-packung