Talk to her again in May. You don't have to choose now. You can choose later. My DD is also an August baby and my friend's son was 4 days shy of the cut off for our district. She red shirted her son. I sent my child forward. We both have had moments of doubt. In the end, it will be like the preschool teachers told me - we'll never truly know because we don't have a clone of that child to observe doing the opposite.
My SS was always older (Dec. baby with a Sept. cutoff) and my SD was always younger (late July). They both fared OK and had their strengths and weaknesses. One of the things I did with DD was send her to a summer session that the school offered before I made my decision. She did OK there, so we sent her on to K. Even had I not sent her to K, the reality was her preschool teachers didn't think she'd be challenged there, so I would have had to have found a new program for her.
You are already worrying what will happen a year from now. Start with seeing who he becomes in a few months. If over the course of the summer you don't think he has the maturity (which I think is the biggest thing - my DD is smart, and my friend's son is super smart, but we both worried about the kids' maturity) to go to K, then find another program for him. But do listen to what the preschool teachers have to say. This is your son, not your niece. If you think he will struggle, why? What are your specific concerns and can the preschool help you address them?
Could you talk to the kindergarten? See the "norm"? There's a wide range of abilities coming into K, even if what they "want" is a kid who does x y and z. I bet he's not as far behind as you think he is since he's been in preschool. And as far as writing, there are things you can do to encourage that, but bear in mind he's only 4.
Overall, I don't regret sending my daughter to K. We thought hard about it. She has her moments of behavior which could be better, but she's flying through the reading and is already at a first grade level (and she met her first sight word in the elementary school's summer program, not in preschool). She would have been bored to tears in preK. So we work with her on the areas she needs to manage and praise her for her successes. Every kid has his or her issues. Even my super academic, super well behaved SS had problems with things like time management his whole public school career.
So....unless there is a huge maturity problem, my opinion is to send him on if the preschool says he's ready. See what he can do. If you do hold him back, do it for his own sake, not because it's currently trendy or something. Talk to HIS teachers and HIS schools and really think about HIM.
ETA: If you feel he could use some work on motor skills, talk to the teacher. There's a lot you can do before August.