Saving copy of 10.5 MXD to 10.4 using ArcPy at 10.5?
I can confirm this. I have 10.5 installed and I also do not get the option to save as 10.4.
However, there is also no option for saving as 10.2. This is because MXD documents saved in 10.2 can be opened in 10.1.
My first thought was that documents saved as 10.4 can be opened in 10.3. But I was able to test this and it doesn't work.
So, it's likely that this is a bug. I suggest you log this with Esri.