F100 w/SB-28 in shutter priority, normal TTL- why aperture stays wide open?

...with AF85 1.8D, matrix metering, single.

Do I have to be in manual mode to drag the shutter a bit (20th-30th)

at a given f-stop, f4 in this case?

In shutter priority the camera chooses the aperture according to what you have chosen for the shutter and the ambient light. If you go to a slower shutter speed the aperture will stop down, but that's not the right way to do it. Go to manual and set your aperture then your shutter (from 1/250 on down) according to the meter. The flash will take care of itself.

Try aperture prriority instead with the sync mode as either slow

sync or rear slow sync mode for the speed light.

In shutter priority the camera will set the f-stop at f4 and let you set the shutter speed between 60th and 250th.

In Aperture priority it will set the shutter at 60th and let you set the f-stop where you want it.

 

For these reasons, I always use manual mode and set each where I want them, looking at the back of the flash to ensure my subjects are in range.

 

For slower than 60th, you need to set the flash to slow or rear curtain sync.

 

Read the manual!

If the camera is set to manual mode then you do not need to set the flash to rear or slow sync. Aperture priority and slow/rear sync may yield very low shutter speeds, because the camera will try to set exposure based on ambient light levels. For flash with shutter speeds in the 1/20 to 1/30 sec range, set the shutter speed and aperture manually.