STM32F103rb datasheet mentions 4gb address space, while only 128kb flash and 20kb RAM?
It is the same than with your PC really. It has 64-bit address space which supports 16 exabytes of memory, but you only have maybe 32 gigabytes of memory actually installed.

The STM32 memory is divided into few bigger areas like Flash, SRAM, factory boot ROM and peripherals. While each area is very large, it might only contain a small area that is actually used, and in case of peripherals, not all addresses are contiguous.