Hi Jones,
You have teen told incorrectly. Natural latex has different properties than synthetic latex and while the overall properties of natural latex are superior in many if not most ways to synthetic latex … synthetic latex also has its own strengths that are superior to natural latex in some ways. There is more about the difference between natural and synthetic latex in post #2 here.
I would also be careful believing statements that are overly general because ventilation and airflow are more important in regulating sleeping temperature than whether a material is natural or synthetic and the method used to produce the latex will have more effect on temperature than whether it is natural or synthetic. In general Talalay latex is more breathable than Dunlop regardless of whether it is natural or synthetic or a blend of the two because it allows more airflow through the material. There are also other factors involved in the sleeping temperature of a mattress (such as using wool in the quilting layers) that may be even more important than just the type of foam used which you can read about in post #2 here.
There would be no real reason for a manufacturer to add 4% SBR to their latex other than as a method to improve it’s consistency because the difference in cost, performance, safety, or durability in using such a small percentage of SBR would be insignificant IMO.
Phoenix