I use a Panasonic Lumix DMC-FZ1000 but if you want to use your canon glass, the Panasonic Lumix DMC-GH4 with an adapter is probably the best option.  Lots of interesting reading here and in their forum if you want more options.
http://www.eoshd.com/

Depends what you want to be able to do, what you already have, and your budget.  Use your Canon lenses?  Have high quality audio?  Autofocus?
I would stay away from point and shoots if you want quality, shallow DOF, easier manual focus, etc..

can you keep stuff in focus with those cameras? I have a hard time shooting video with my Canon 5dM2 because I cannot see the focusing on my LCD with my 65 year old eyes. So I got a SONY HXR NX3. It does not do 4K but at least stuff is in focus.

I am not really into 4K now. Just to sterile. Good for landscape aerial i guess. Might change... dunno. Plus my UL bandwidth sucks. Hard to get even small files ULed to SS.

can you keep stuff in focus with those cameras? I have a hard time shooting video with my Canon 5dM2 because I cannot see the focusing on my LCD with my 65 year old eyes. So I got a SONY HXR NX3. It does not do 4K but at least stuff is in focus.

I am not really into 4K now. Just to sterile. Good for landscape aerial i guess. Might change... dunno. Plus my UL bandwidth sucks. Hard to get even small files ULed to SS.

Yep I have a 4k camcorder but I'd choose something like this before a dslr just for ease of use.  It requires a lot less set up and having built in ND filters is a real plus.

The FZ1000 did a great job on tracking focus.  I didn't miss anything.  And I used a $28 ND filter from BHPhoto, shooting outdoors.  It did have a variable aperture across the zoom range, but it wasn't a huge deal.  I would have liked a tighter DOF in some places.

4K isn't the only thing the FZ1000 is good at. It can compete with high end SLR's if you know how to use it and more importantly know to process the pana RAW's for noise and sharpness. If you're not good at PP then you might think it's just ok for stills.
f2.8 to f4 on a 25-400mm lens is not too bad. Try to find one with that range for an SLR. Sweet spot at f4 and 5,6 depending on subject matter.
This is a one of a kind, a hidden gem amongst the camera's out there.
I do wish for manual zoom/focus though on the next version to save on battery.
Have fun with it! learn it well and like many others you might find not much use anymore for your other gear.

I plan to start to make stock videos from the beginning of next year, and feel that I should go straight to 4K.

My 5Dii only does HD I believe, so for 4K I'll have to invest in new equipment.

What equipment do people here use to produce high quality 4K?  What's the best to buy in terms of the best balance of budget and results?

It seems like you should try doing stock footage with your current camera and see how it goes before trying to invest in another camera.

Just about all sales are still HD. I had a 5d2 and they can produce very decent video.

New iPhone shoots 4k video.

Yea, thanks Apple. My wife wants a new Iphone now

I do like the GH4 with Metabones adapter because I can use any of my Nikon lenses.  It produces very sharp video with Cine settings.  But there is an investment of something like $2500 and that does not include lighting if you plan to shoot in a studio.  I have shot video at 800 ISO with that camera and if the exposure is right you can't really tell. Very flexible. Oh, and you will need some good ND filters to control light.  My most used ND filter is an 8X for outdoor, daylight shooting.