From the kernel 3.4 code,I could not make out that where the UART Base address is mapped ?As far as I know for mapping the base address we should use request_mem_region and io_remap function.But I could not find this in the code.Base address for UART4 is 0x4806E000
This should be defined in the dts file in arch/arm/boot/dts/omap4.dtsi file. I'm not sure if DT is enabled for your kernel.If enabled, it reads in the probe function