I think 4 years old is a reasonable age to check things out more - a little early to get some definitive answers, but good to be aware of things. On some things, we were able to get definitive answers that early. We noticed a few things my son was struggling with (he was in a play based preschool when he was 3 and 4 years old). He had an Occupational Evaluation at 4.5 years old, and confirmed some of what I suspected. Some of the other things I was concerned about, we were told that it was too early to tell if they were issues yet or not - that most kids his age were able to do those things, but not all and so in order to see if he was at the very slow end of the average range, we gave him more time to develop some things.
He started OT shortly after the evaluation and moved to a Montessori school this year, both of which have helped him tremendously. DS also saw a psychologist (who said that at 4 years old, he was too young to make a diagnosis for ADD or ADHD). We also considered seeing a developmental pediatrician.
I actually wish I had followed my gut and pursued some of this stuff earlier than I did. I have also found his Montessori teachers much more aware of, and able to address, the areas he struggles with than the teachers at his other school.