the very average image quality Canon EF 16-35mm f/2.8L version II was introduced in 2007 this has been updated in Dec 2016 with a mark III version which is sharper wide open at 16mm and with less CA and the 9 blades give much better sunstars but otherwise fairly similar at other focal lengths
compared to the 16-135mm f/2.8L II lens, the newer Canon EF 16-35mm f/4 L IS is:
sharper edge-to-edge
less expensive
adds image image stabiliser so you can shoot at hand-held at 1/2 to 1/4 second
specs
Canon EF 16-35mm f/2.8L III USM
announced Dec 2016
Fully weather-sealed construction
Sub-wavelength Coatings (SWC) and Air Sphere Coatings (ASC) to eliminate lens flare and ghosting
Two large-diameter double-surface GMo lenses plus a ground aspherical lens to help decrease distortion, curvature of field and astigmatism
sharpness wide open highest at 16mm (much better than mark II) but 2.6EV vignetting in corners and 2.7% x 0.9% barrel distortion, but lateral CA well controlled and much better than the version II lens
at 28-35mm need f/5.6 for maximal sharpness and at these focal lengths, the distortion is pincushion, and the sharpness is only minimally better that the mark II lens
distortion similar to mark II lens but simple and easily correctable
Canon EF 16-35mm f/4L
announced 2014
“by far the best ever from Canon for nature, landscape, interior, real estate, general architecture and outdoor photography” 1)
has image stabiliser of 5 stops allowing hand held shots at 1-2 seconds courtesy of the Olympus cameras (the Canon has no IS)
has eye detection autofocus for portraits
has fast, silent AF for movies
why would anyone buy the Canon 16-35mm f/2.8 lens then?
if you already have a Canon full frame dSLR with 50mp and you shoot landscapes on a tripod at f/8 then it may have a use as it can take filters although you should be buying the much better f/4 lens (although for static scenes, the new Olympus cameras can shoot 40mp HiRes shots with less moire on a tripod)
one could argue you can get 2 stops better ISO on full frame, but with this lens you need to stop it down 2 stops to get close the the sharpness of the Olympus lens, so that argument generally won't hold much