External trigger is commonly omitted on four-channel scopes. Even the four-channel Tek scopes I've had don't have it. I guess it's seen as something of a hack to get a bit more performance out of an otherwise limited two-channel instrument...
That's actually interesting, because the page 18 of the manual (from:
http://www.batronix.com/pdf/Rigol/UserGuide/DS1000Z_UserGuide_EN.pdf) says:
"Chapter 5 To Trigger the Oscilloscope
Introduce the trigger mode, trigger coupling, trigger holdoff,
external trigger and various trigger types of the oscilloscope."
Of course, the chapter 5 doesn't describe external trigger! (and the instrument doesn't have one).
There are more such gems in the manual, e.g. the option to switch the SPI/I2C decoding source to "Memory" instead of just screen - the option does actually exists on the instrument, but is permanently greyed out.
On the other hand, if you have 4 channels, it is probably not a too big pain to simply use one of the channels for triggering. On a two channel scope it would have been more limiting.