Built and tested Evolution with stable webkit-gtk:4. Not a Gnome user myself but checking shows all other stable Gnome 3.18 packages are using slot 4 while Evolution was the only package on my system depending on webkit-gtk:3. (Out of scope of this bug but probably all webkit-gtk:3 dependencies could now be updated to webkit-gtk:4 and webkit-gtk:3 retired.)
This is not how it works unfortunately. evolution uses webkit-gtk:3 and it not compatible with :4 at least in 3.18. This is true for a couple of other package and this is why they depend on different slots. When there is a choice we always select the latest slot and try to get rid of the older ones.