I was thinking about this today while listening to my 4-year-old speak, and it really made me realize that if you were having trouble understanding *my* 4-year-old (who is probably really close in age to this little girl, as she won't be 5 until the end of October), I would really appreciate knowing it. My daughter's speech isn't as good as her older sister's, at 6, but I do understand pretty much every word she says, so I'm not sure I'd notice if other people could or couldn't, you know? I like to think that one of her teachers at church or AWANA or Mother's Day Out would mention it, but who knows? So I'd definitely appreciate knowing so that I could get it out.
I do think that it's something that takes a person spending some real time with the kid to realize, though. I was listening to my daughter today when she was on the phone with my mom. My mom has mentioned having some trouble understanding her on the phone, and I figured out why today. When she's on the phone, she gets really excited, moves around constantly, turns away from the phone or sometimes forgets to hold it to her face, refers to things that are in front of her as if my mom can see them (like picking up a toy cell phone and saying, "This is Charlotte's. Sometimes she talks into it," when my mom has no idea what she's talking about). I could definitely see a kid that somebody only sees, say, at crowded noisy family gatherings, or at school dropoff when the kid is really excited, or at Zumba when the kid is practically a zombie and overdue for a nap, having trouble understanding an otherwise understandable kid. But in your case, it sounds like you see plenty of the girl and have the time (and a bit of experience) that makes you somebody who can truly notice something that might be a real problem in time to help the kid. And nowadays, needing some speech therapy isn't shameful like it was when I was a kid.