If they don't get something out at Photokina to compete with the GH4, I suspect most of the low budget indie crowd will consider Canon dead in the water, and start migrating, unless they've already got too much invested in glass and already have a 5D Mk III.
The GH4 is certainly an interesting camera, but not perfect. A lot or most Indies cant afford or handle a full 4K workflow yet, shallow DOF is a challenge on the GH4 since you need much larger aperture lenses to achieve it (think f/1.2 ), XLR is nice, but most people use separate audio and it's not that difficult, some people like Philip Bloom have reported that low light performance is not that great (5D3 is awesome in low light). The GH4 is no longer small and light, and with the Audio option, it's quite large and heavy, and yet still not a good option for ENG ... and it's $4000 . I'm not tempted by the GH4. The 5D3 footage still has a special beauty to it.
The real threat may come from the Sony A7s ... if reviews confirm the promise. Small, light, full frame, unbelievable low light, and 4K available for a high end production if you really need it. I'd like to see Canon put something head to head with the Sony A7s rather than the GH4