it's probably 4 uses, not casts, i.e. doublecasting from spell echo would probably only grant one stack. 'Using' a skill generally refers to the initial activation, not any repeats (or any continued channeling, in the case of channeling skills).
And 4 casts may not take very long, but when you drop all stacks from just brief movement to dodge and have to build up again, you're cutting down your average damage, by a lot.
Conc effect would give you 54% more damage all the time without buildup; for intensify to average that level of dps amp you'd need to use a skill 16 times in a row without breaking your intensity... the ramp-up kills your average dps that much. Forgive me if i don't think enemies are gonna wait around long enough for you to make that pay off.