The attached image shows a sample 4D image that you can replicate with File/OpenStandard/pcasl

1. The quick way to choose a volume is to click on the volume in the timeline. The timeline is the bottom panel - it is collapsible so make sure it is visible.

2. You can use a script using the `volume()` function. You can see all the functions with Scripting/Templates/help menu item. The script panel is on the right, so expand it if is collapsed. Like other tools, volume number is 0-indexed, so the 60th volume is 59. For me, the volume() does not refresh the image so I use the orthoview() function to force a refresh. So a script would look like this:

import gl
gl.volume(0,3)
gl.orthoviewmm(0,0,0)