A 16th note is a quarter of a beat in 4/4, so therefore the popover syntax is: 4/4,0.25
In 4/4, some example pick-ups are:
- Quarter note/crotchet = 4/4,1
- Eighth note/quaver = 4/4,0.5
- Half note/minim = 4/4,2
See also this page, which now has more examples, including a 16th note pick-up in 4/4