4% extra damage means that the damage dealt is multiplied by 1.04. It’s as simple as that. Normal hits do 100% normal damage, and crits (pre-meta gem) do 200%. Multiplying each of those by 1.04 gives 104% damage on non-crits and 208% damage on crits.
Yeah dps or tank for 2h some talents in blood is good. Depending on build maybe up to dark conviction more if you are a tank, less if you are frost dps. As for talents % based talents like 1% more crit or 4% more 2h damage would scale well with gear.