The Dan Hollander article is excellent. All parents should check it out.
I would think twice about someone who wants to evaluate potential in a four year old. I once tried to refer a kid I had been coaching to a more experienced coach that I respected. I felt she was reaching the maximum level I could realistically coach. His response was that she was already 8 years old and no axel, so he would give her lessons but wasn't enthusiastic. I made the referral to someone else and lost respect for the guy as well. Few kids have inate "talent", but they can still benefit from lessons. besides learning to skate, lessons for a child so young should stress fun, working at something pays off in results, goal setting (all adapted to the childs age), etc. Not every child will make it to Nationals or even regionals, but they all can learn valuable life skills thru skating, not to mention fitness, posture, confidence, etc.
Any parent thinks their child's coach is great, so asking parents may not be valid. Instead watch the coach interact with their students. At our rink, one guy is constantly yelling (angryly and in two languages)--I would never recommend him on that basis alone. Sometimes you do have to yell, but it should not be frequent, nor should it be derrogatory or sound angry. Another coach occasionally yells, but often he is yelling technique reminders, encouragement or kudos for a good element or good effort. His skaters are always smiling when they talk with him. This is the guy I'd want my kid to work with.