Yes, you just described stacking, i.e. adding two skills on top of each other. Gear 4 not only gives you the strength of Haki, but it uses Haki in tandem with his DF, to improve his physical strength.
That's where the phrase
greater than the SUM of its parts comes from. You are using one skill to make another skill better.
If Luffy's base strength is 10, and his Haki is 10, by stacking you get 10 + 10 = 20. But if Luffy harmonises them (multiplies), you get 10 x 10 = 100.