Way to go! Eyes at the level of a 4 year old would get an awful lot of 'stuff' thrown at them. (I'm not implying you'd put your son in front of a wall of wood chips flying out of the saw.) The ANSI Z87.1-1989 is a standard that's followed for eye protection. (Every day eye wear is usually only impact-resistant.)
I guess the way I'd approach it is I would put the same type and quality glasses you wear, or better, on your son. Consider side-eye protectors. Maybe an adjustable hard hat from Santa would be in order. 'Stuff' at his level can fall off of tables and such too. ;-)
My best to ya and hope this helps.
PS: Check out What's New From Titmus. Many of these comply with the ANSI Z87.1-1989 Standard but they look better suited for adults. It's just here for your information. If you go to your favorite WEB Search Engine, and enter in Z87 and ANSI and eyewear, you'll get a lot of 'hits'.