M4 appears to be an old male in many ways, mainly through his behavior and demeanor. But, I've noticed that he has a lot more white feathers than the other adult birds. On his face, he still has much of the black that many males have, but he does have a LOT of white feathers there. Also, his tail, which should be mostly brown, black and orange has a lot of white, also. This is not normal as the tail is the way that males display threats not only to potential predators, but to other males and to court mates.
Today, when I visited M4's territory, I noticed that his voice was very, very raspy. He can still do his high-pitched "kill-dee" call, but all his other calls sound like he has laryngitis. He could just be sick, but his over-all feather appearance and health appears good. He doesn't have any bare spots or feathers sticking out strangely--a sign that a bird is not healthy. His coat generally appears glossy, though not as glossy as the very young male, M1 ("Little Male").
The beautiful, young female in M4's territory seems to be gone and that could be why he spends much of his time flying around and doing territorial calls. It's very possible that he's in a lot of fights, but his feather condition doesn't indicate that, as well, except for a little scruffiness.
I can't really estimate M4's age as I have just met him this year, but killdeers have been known to live 10 or 11 years in the wild, but not usually. I would suspect him to be a minimum of six to seven years old, possibly eight or nine, but not beyond that. If I am correct about his senior status, this could be his last breeding season.