The four 'new' Mr.Men books and nine Little Miss books were published after the death of Roger Hargreaves.
Is it possible Mr.Hargreaves was writing and illustrating new books prior to his death and they were published posthumously? Yes. Possible. But the last Mr.Man book (Mr.Slow) was published in 1978. While Roger Hargreaves wrote and illustrated the Little Miss books and many Mr.Men companion books after that, no new Mr.Men were published until 1990. 12 years later. Did he just hold on to four books? Seems unlikely.
Prior to 1990, the last Little Miss book was published in 1984. And then nine new books appear after the death of Roger Hargreaves. If he had been working on them and the series remained popular, why wait until after his death?
Why is this a big deal? Because the books say 'by Roger Hargreaves' on the cover. And give no indication anywhere else that they weren't, in fact, written and illustrated by Roger Hargreaves.