Three solutions comes to mind:
(The four units are named u0, u1, u2, u3)
1) Make a trigger with West Present, in the Condition field write:
this && ("_x in thislist" count [u0,u1,u2,u3]) > 0
2) Make four triggers, West Present, then group each one with one of the units. (non-scripted approach)
3) Make a trigger, 0 radius, (or have it spinning in a script (with @<condition>)), in the condition field write:
"_x distance safe_obj < 50" count [u0,u1,u2,u3] > 0
Where safe_obj is an object in the center of what you feel is safe-ground.
None of the syntax is guaranteed.