Blue Moon -- you are truly all alone in this.

Combustion temps are related to a bunch of things, which include compression ratio, ignition timing, induction timing, exhaust timing, squish area, combustion cycle, fuel, air-fuel ratio, load & heat rejection. There is too much variability in conditions for a sweeping statement to have any validity.

What is relevant to the question asked is the sustained temperature in the exhaust system, & 4-strokes generally have notably higher sustained exhaust temps.

If you doubt it -- grab the exhaust header of each type (be sure to chose engines making similar power) & let us know how it goes. Many of us can tell you from first hand experience [], but you do your own thing.