The sentence "Most were first-time users of a tablet and a digital app." is most logically placed after Sentence 4 in the paragraph. In Sentence 4, the author provides a clear contextual reference by mentioning that the app was piloted with over 400 women. This preceding sentence sets the stage for the information about these women being first-time users of a tablet and a digital app, making the sequence 4 the most appropriate placement for the given sentence.