Hi dn, Clawdia, and Jones,
The properties of natural rubber like any natural product is subject to many variations based on weather, growing conditions, and seasonal variations. Synthetic rubber on the other hand is a much more consistent material and can be chemically formulated to have specific properties.
My best guess would be that they use 4% synthetic rubber as one way to fine tune their compounding formula to help make the natural rubber more consistent batch to batch and year to year.
I would also be a litle wary of buying a mattress that used a material that I wasn’t familiar with so I would at least test a mattress with similar materials and design so you have a sense of how the material feels and performs. In a perfect world you would be able to test a specific mattress but if you are buying online then the options you have available after a purchase to change or fine tune PPP (Posture and alignment, Pressure relief, and Personal preferences) may become one of the most important parts of your personal value equation.
Latex is the most breathable of all the different types of foam but all foam is an insulator compared to natural fibers or springs so the sleeping temperature of a mattress really depends on how well the materials in the mattress work together to allow ventilation and wick and transport moisture away and on the overall insulating properties of the mattress. There is more about the factors that can affect sleeping temperature in post #2 here.
While most of the research on the forum is North America based … post #9 here includes a link to a list of some Australian manufacturers and retailers and to some of the Australian threads on the forum that may be useful.
The foundation post here includes guidelines and links to more information about a suitable base for different types of mattresses and post #10 here has more about the choice between a solid surface base vs a more breathable slatted base.
Phoenix