4D does not specifically recommend one Operating System over the other, however there are some obvious differences in the level of hardware that is available between the two camps (Mac vs Win).

On Windows, you can configure a machine that has much more horsepower - such as a dual socket XEON 10 core (20 thread) machine (20 cores / 40 threads total), with super fast PCIe SSD, and over 200 GB of RAM - that could handle much much more than any MacOS could just because of the hardware alone.

On MacOS, usually people deploy using a Mac Mini as their server hardware... which can be a bit limited. The best you can do on Mac is use a Mac Pro with 6 cores or 12 cores and 64 GB of RAM, but usually people do not put those in a server room, it is the designers and professional video editors that use those machines. Even with a Mac Pro though, you can configure a Windows machine to have more cores/RAM and a faster disk IO.

So looking at the hardware alone, Windows is a clear winner for serving a database or application server.