There are a few more than 4.

Technically, sure. But practically — what’s your estimation how many C++ programmers target anything besides these 4, gcc, clang, vc++ or intel? My estimation is [ 0.1% .. 1% ]

Plenty, for example HPC tends to favour IBM's XL compilers.

Embedded tends to have their own vendor forks, even if based on GCC/clang, it isn't the same thing.

Then we have game consoles as well.