The quickest way to enter this is a double-dotted quarter note!
Another way: After the quarter on beat 4,
T for tie, 5. (or 55 quickly) for dotted eighth.
Another: Enter a half note on beat 4, set the grid to a 16th and either:
- Shorten the half note with ⇧⌥←, or just
- Back up a 16th with left arrow
… and you’re ready to enter the 16th note. The latter overwrites the end of the half note.
For reference (and more discussion and argument), this same issue was brought up last week and again yesterday.