With ubuntu 16.10 (or, more precise, the newer Kernel with it) it is a bit easier, the following modes are available:
4096x2160 (0x127) 556.720MHz -HSync -VSync *current +preferred
h: width 4096 start 4104 end 4136 total 4176 skew 0 clock 133.31KHz
v: height 2160 start 2208...