Hi Gediz,
thanks for your answer.
Regarding R_UART i didnt find anything which did help me with UART4.
The problem not only exists with UART4 - but with all possible UARTs.
If i activate UART2, UART3 for example there is also no entry found in dmesg for ttyS2 or ttyS3.
in dmesg always there is this entry for ttyS0... :-/