Buy the 3 now IMO.
These high end camera have about a 3-4 year life time so we are only about half way through the 3's shelf life. It is possible they will release a small upgrade camera like Nikon D800 to D810 but canon has less of a track record for doing this, they tend to try to push for tighter release schedules. Realistically photokina next September looks like the most likely release date [EDIT: at the earliest for the mk4, if they release a small upgrade then perhaps CES in January availability in Feb-March].
The other thing that looks certain and has been more or less confirmed is canon will release a high resolution FF DSLR to compete with the D800. Some rumors about a 5Dmk4 might be about that camera instead.
Lastly you have to think what will really change with a new 5Dmk4. Maybe a small AF bump but the AF is already first rate so I doubt things will change that much from a user perspective. Depending on the resolution of the high MP camera they release they might get a resolution bump (e.g. 32MP for the 5Dmk4 and 48MP for the high res body). However recently canon has been very conservative with resolution increases which is not like them at all.
People have been talking about amazing increases in sensor performance but the only way I see that happening is if they pay Sony, Toshiba, Apatina, or samsung etc for a sensor or technology like new ADC. Canon just hasn't shown much sign of changes in years. Sony didn't suddenly come out with amazing technology, they reiterated and each new generation got a little better. Same with Samsung. Canon are throwing out new EF-S sensors with the same performance while with Nikon/Sony it was the DX cameras that were getting better sensors and even surpassing the FF sensors in some respects since the crop sensor camera are released much more frequently with newer tech.
So I wouldn't hold your breath for amazing sensor changes from no where.
EDIT: typos and clarification