I ran out of Abri-Form M4's with the plastic back and needed to find an alternative. I thought I'd share my findings with others who are maybe in the same position.
I ordered from Save Express in Germany and bought a variety of alternatives:
Tena Slip Maxi
I knew these are reliable and have worn them before. They're still plastic backed and have excellent absorbancy.
NEU Kolibri Comslip plus
These are being marked up by Save Express as being the Abri-Form replacement. On first impression they look very similar and do seem to be a very good alternative to the M4's.
Forma-care X-PLUS Slip Gr.M , Nacht , blau
Also very good, on first wearing they're very comfortable and didn't leak. I think these are my favourite alternative so far.
Kolibri Comslip Supra Gr.M
Cloth-backed, I bought these as their absorbency was rated at 4000ml which seemed a lot and therefore worth experimenting with.
I've only tried one of each so far, so it's too soon to decide what I'll re-order. They all seem pretty comparable to the M4's though. Mostly the same size and comfort, not quite as thick as M4's but as effective and leak-proof.
I'll post an update once I've formed a better impression :-)
What's everyone else moved to in the absence of M4's?