The replacement to the EF 100-400 f/4.5-5.6L IS is coming soon at the end of this summer. Canon EF 100-400mm f/4-5.6L IS II lens rumored to be announced at the end of August, 2013.
According to CR, a few prototypes of the lens are currently in use in Asia. The current EF 100-400 f/4.5-5.6L IS lens is on the market since 1998.
Canon EF 100-400mm f/4-5.6L IS II lens to be Announced August 2013?
After Nikon released their 80-400mm VR AF-S lens, Canon thinks to respond quickly with a new replacement of the lens. Looks like Canon is going to be very busy in the second half of 2013 with new lenses and DSLR cameras.