If paging is disabled and the current segment's limit is 4GiB (in 32-bit mode) there are no "nonexisting" addresses:
All 2^32 possible addresses exist in this case and can be read and written.
What happens if a read or write operation to an address where no RAM, ROM, etc is located is done depends on the hardware outside the CPU and not on the CPU itself.
A write operation to such an address will typically be ignored and a read operation will typically result in a non-sense value (on most PCs the "all-ones" value like 0xFF, 0xFFFF, 0xFFFFFFFF).
Theoretically such an address access may cause an interrupt or even crash the computer depending on the address. However this is not done by the CPU itself but by other hardware components.
Execution of code on such an address is basically nothing but a read access from that address.