@Dino,
How to disable the memory regions already allocated ?.
Solution: what i think is that you can disable UART driver from kernel (menuconfig), build the kernel image and check /proc/iomem, I guess UART4 related instances will be removed.
Note: If using device tree concept then need to disable UART4 node "@uart4" in device tree.
I would like to use it in my driver code with different name. Is it possible?
Solution: In your LKM request for the memory range using request_mem_region with your driver name.
BTW: Why the call to ioremap is not failing ?
Solution: ioremap api will not fail because api does not have check for ex:"driver name" or any other checks.