The fact that the smaller model is more expensive makes no sense in and of itself. It is likely that the X3500H is the list price when the model was still current, whereas the X4500H has been priced to reflect that it is the previous year's model.

Could well be, I also thought the X4500H should be the "better" one^^

Unfortunately, I can't answer this definitively either, but I refer you to the page https://www.zkelectronics.com/compare... (English), where both models are compared.
The way it looks there, the X4500 is the better equipped and more powerful receiver, so it could be a good deal if it's cheaper.... Maybe the production got cheaper in the meantime or something.