Right now I have no plans to sell mine and go to the f/4 IS version. Have to admit I'm waiting to see if Sony/Zeiss can get it right with theirs for the A7R. I'm trying to turn that into a landscape camera. I use my Canons as the go to camera for my event, sports, wildlife, portraiture, and macro work. It is still my main landscape camera as well so the 16-35L II does get used. I am trying the A7R out for landscape with it's higher resolution for large prints and possibly travel but without a good 16-35 IS it will never happen.
Their 24-70 was a real bust in some ways but they did keep it small. Don't know how it compares to the Canon 24-70 f/4L IS. I know its can't touch my Canon EF 24-70mm f/2.8L II which I love.
I admit I own too many cameras and systems but I'm trying to find a camera for recreational travel that is lighter and small than my 1DX and gripped 5D III. Right now I'm using a Fuji X-E2/X-T1 combo even though its not FF is offers a great group of lenses. Their 14mm f/2.8 (21mm equivalent) and 10-24mm f/4 OIS are both great lenses for an APS-C system for landscape and architecture.
So I'm in no rush to change anything.