From the 1st page of this thread the X4100 can process a maximum of 9 channels in Atmos - even adding more external amps would not help to achieve the following speaker configurations.
[*] 7.1.4 (default recommend by Dolby)
[*] 9.1.2 (using wides in DSX or Neo:X)
That means the for the X4100 owner he/she is limited to the Atmos speaker configuration of 5.1.4 (default recommend by Dolby) or 7.1.2 (with top middle).
For those who have an existing 5.1 speaker setup and would like to experiment with Dolby Atmos, going to the X4100 makes a lot of sense.
People who have a 7.1 setup and would like to have more flexibility in speaker arrangements means stepping up to the next higher model (X5200) or the highest end X7200.
Marantz SR-7009, Oppo BPD-93, MTV 7000D, aTV, Harmony Home Control remote, KEF E301+T101, MiniDSP (2x4)+(10x10HD)+(DDRC-88A), Emotiva 2xXPA-5 (Gen2), Rythmik 2xF12G+2xF8, HiMedia Q16, LG 55EC9300.