Need help on mulch job tomorrow

I don't spread much mulch but have a customer with 3 larger areas to put some out. They said they want it about 4 inches thick. And then mentioned something about digging out a groove at the edge where it meets the yard with like a half moon edger. Just seeking suggestions if something like this is needed and or can you slice out a line with the weed eater? Will the mulch get into the yard when it rains if none of this is done? Thanks much.

Brian