Design and Display
The Canvas 4 has a 5 inch touchscreen while the Canvas Doodle 2 has a way big 5.7 inch display. However, screens' of both the smartphones have the same 1280x720 pixels resolution. How much that affects the screen quality can only be ascertained once we have the Doodle 2 in our palms but prima facia the Canvas 4 seems to have a better screen while of course the Doodle 2 has a big screen - so its like quality vs quantity.
As of now, the exact nature of the Canvas Doodle 2's design is not known but since it has a huge screen it is ought to be heavy and thicker than Canvas 4.