very simple if you use a cuelist
set cuelist to internal or relative or timecode (as appropriate)
Program a preset for 'Cue 6' (looping movie layer 3)
Program a preset for 'cue 7' (new movie layer 4)
Put 'cue 7' preset in cuelist to trigger 250frames after cue 6
There is no 'triggering' feedback from layers - you must use a cuelist to automate as required
Any queries - do get back to me
Simon