How do you know if you need to hire a sleep coach for your preschooler (4 year old)?

Has anyone ever hired a sleep coach for a pre-schooler? Our 4yo has never been a good sleeper. We finally got her to go to bed at the same time as the big kids (instead of falling asleep in my wife's lap in the living room later), but my wife has to lay down with her until she falls asleep and then ...