Make mosters spawn randomly from 4 spots

I have 4 spawning pads set around my layout, and I cannot get the monsters to spawn randomly between the 4, even if the number of monsters is being gradually increased.

I tried what I thought would work, and even though the syntax was correct, it didn't do anything. Thanks in advance.

See scirra.com/forum/random-spawn-location-set-number-of-spawns_topic72116.html

Would it be possible to just have monsters spawn randomly outside of the layout, without spawning pads and without the need for 20 extra sprites?

for that use something like spawn at random X, random Y