RE: whats the smallest 4 stroke?
Well... how are these for small? Check out the pics! They are hand made by a master craftsmans. If I remember right, the inline 4 is .061, or so. I know the 5 cylinder radial is .061. I don't recall the size of the single but must be very small. They all work too! I came across these while on a search. I don't know the site, but if you search for Luhrs you might find them. I know I would love nothing but to have that inline and radial for any of my planes. Specially a Gypsy Moth or any antique scale model. Don't think I could afford them if they were for sale though.
ZZ