Child spacing - just wondering?

I am 35 married for 10 years. We have a 4,2,and 1 year old. The last child came upon us quicker than I anticiapted. We are wanting to ahve another. Our oldest has special dietary and physical needs and we've chosen to send him to private school about 20 miles away (each way) but we already do this 3 days a week and he gets pt 3x a week after school. Our other two children are typical and come along or stay with grandma when we do his pt(physical therapy). They can also particiapte in it it is specialized swimming lessons and typical siblings are invited to participate at age 3. Should I wait another 6 months a year or not at all? Any ideas on spacing and my age and children's diverse needs?