You have a newborn, so its understandable about "germs" and the infant getting sick... per your older child if he switches schools.
Now... if you feel he is not being challenged enough at his current Preschool:
1) you teach him things at home. I did that with both my kids, and still do... even if my kids went to a Preschool. I still do that now, and my kids are in Kindergarten and 4th grade.
2) Thus, you can keep your son at his current "preschool."
3) Then, when are you sending him to Kindergarten? In Kindergarten, regardless of where a child went to preschool or what they learned there... Kindergarten is going to be a learning thing for the child too. AND they will all learn there. Even similar things to Preschool.... but more complex perhaps, depending on the Kindergarten curriculum.
4) Your son, WILL be attending a 'Pre-K' in Sept. So that's fine. Perfectly fine. He will learn other things in Pre-K.
Whether it is Pre-K or Kindercare or a play-school environment... a kid learns things. Then in Kindergarten, they will learn other things.
Being he is 4... the main thing is not only "academics" but socialization and learning to follow direction, and structure etc.
It is not only about "academics" in other words.
I wouldn't fret about this.
Just keep him where he is.
That's fine
Just keep your son where he is.
But keep in mind, that at any new school, kids tend to get sick... a lot, until their body gets used to a school's germ atmosphere. This goes for adults, too.