Mercury renamed most of their engines they used to call OptiMax engines to be called ProXS engines.
On the
website for Mercury Marine, I found a mention that the ProXS 115-HP model has an overhead cam. That makes no sense as a two-cycle-engine. I guess we have another situation of Mercury using some confusing nomenclature for their engines, that is, apparently the ProXS designation means anything they like it to mean, and ProXS engines are no longer re-badged OptiMax engines.
If you use the comparison tool on the Mercury Marine website to compare the ProXS 115 engine to the FOURSTROKE 115 engine, you will see that every specification is identical. On that basis, I would infer the engines are identical, and the new marketing promotion of the engine as a ProXS engine is some sort of campaign to establish the ProXS branding as moving away from the OptiMax branding and into four-stroke-cycle engines. The inference I get from the ProXS branding is that Mercury is trying to attach some sort of association and prestige from their racing division engines onto their consumer grade engines.
The best differentiator will be the price. If the ProXS 115 engine can sell at a higher price than the FOURSTROKE 115 engine with identical specifications, then the marketing and branding strategy will have succeeded.