Preschool is perfect for him now. It will get him "ready" for "school." It is less structured and usually more allowances are made for the children, because of their age.
Pre-K... when he is 4.5 years old is fine too. But I personally think, that 1 year of preschool is best, THEN put him in Pre-K or Kinder. Many times, a child will "outgrow" preschool at this age anyway.
Pre-K... is usually more structured and follows more of a "school" curriculum, and is located IN a "school." Pre-school on the other hand...is usually located within a "daycare" setting....So there are usually "younger" kids around, and it is more "baby-ish" in that sense.
I've noticed, (with my own child & her friends/classmates) that by the time they are 4 yrs. old & going on 5 yrs. old...there is a big jump in development and maturity and "need" for other stimuli. Thus, my feeling is that putting him in Pre-K at that point (when he is 4.5 yr. old) is more suitable.
In my case, my girl started Preschool when she just turned 4 years old. We had her in preschool for 1 year... then we put her in Kindergarten when she was just almost turning 5 years old. It was perfect for her.... she had had enough of Preschool and was "ready" for "school"... and by the time Preschool ended she was more than ready to start "formal" school, ie: Kindergarten. She even told us that she wanted to go to Kinder NOT another year of Preschool. Her Preschool teacher also said she is ready, and in her case, recommended Kinder for her at that point. In our case...BEFORE entry into "elementary school"...they assess all newly entering children... and for my daughter, she was allowed to go on to Kindergarten... versus "Pre-K."
Yes, having a child, who is ready, enter into Pre-K in public school is "cheaper." And yes, Preschool IS expensive.
The main thing is... your child. Don't go just by "age." Ask yourself AND the teachers IF your boy will be "ready" to go on to Pre-K, both in ability and maturity.
Also though, if next year he is 4.5 years old.... he MAY be able to go on to Kindergarten, versus Pre-K, depending on the birth-date cut-off rules in your area. Like in my daughter's case- My daughter is late born (October), and started Kinder when she was about 4 and 3/4 yrs. old, going on 5 yrs. old. (in our area a child has to be 5 yrs. old between August and December to enter Kindergarten). She adjusted perfectly fine... and had no problem in Kinder. She is now in 1st grade. For her, going to "Pre-K" would have been redundant, and a repeat, because her "Preschool" prepped them so well. Also emotionally and maturity wise, she was "ready." And for her, she didn't want to be in Pre-K or Preschool again... because she didn't want to be around younger kids/babies anymore. She told us this. She was just so excited about being in elementary school.
So think about your options... and your son's "readiness" and maturity.
On the other hand, I have friends who sent their child to another year of Preschool... at 5 yrs. old. By this time, it was mainly because the Parent was not "ready" to move their child on to Kindergarten or Pre-K.... even though the child WAS ready. Then, their child is now in Kindergarten at 5.5 yrs. old, turning 6 yrs. old soon. So it's up to you... some Parents feel that in their case, having their Child start Kinder at 6 yrs. old it better.
Yah, so confusing. But really, go by what you think is best and for your child as well.
Also though, the Preschool Teacher should make some recommendations, if you ask her/him. AND if entering into "Pre-K' in an ELEMENTARY school setting...they usually have assessments done for all newly entering children...THEN your child will be placed in the appropriate venue- ie: Pre-K/Junior-K, OR, Kindergarten.