The four books have remained in print more or less since publication, and certainly since being taken up by Puffin, Penguin's children's paperback division. In 1992, Puffin published the four books together in a single volume, The Adventures Of Milly-Molly-Mandy. The dust-jacket (reproduced here) claims that this is the first publication of all the stories in a single volume.
Pedantic note: while the paperback editions I remember from the 1970s used mildly colorised versions of Brisley's own illustrations (or illustrations in her style) for the covers, this edition features two colour pictures by Elisa Trimby. Quite apart from looking not very much like Milly-Molly-Mandy, the image is unsupported by textual detail. On the cover, Milly-Molly-Mandy is seen setting off on a fishing trip with Toby, the dog. However, in the story Milly-Molly-Mandy Goes on an Expedition, which features Milly-Molly-Mandy and Billy Blunt on a fishing trip, Toby is never mentioned. Still, I suppose it's fair to assume she went fishing more than once.