On top of what has been said, a much simpler explanation. You're having to weave 4 Buffs into one GCD. Blood for Blood, Battle Litany, Internal Release and Draconian-Potion. It's impossible to do this without heavy clipping. This is still a WIP for the guide, explaining why HP ICD4 is worse:



Look at the amount of skills which aren't buffed our 4th Combo in the end. On top of that, due to the forced Drac-Pot placement, Phlebotomize isn't being buffed either. You'll want Full Thrust over Phlebotomize as it's 10 Potency Higher.

H P IDC4 has 1835Potency * 105 STR (Pot on ID)

H P IDC4 has 2045 * 105 STR (Pot on DI)

H IDC4 P has 2105Potency * 105 STR

You could also use the Drac-Pot on H IDC4 P before Chaos Thrust, like the "Advanced" opener in the thread OP, giving you an additional 70 Potency (WT > DI)

And not only that, as IR + BL aren't buffing your Wheeling Thrust, that's around (1+0.5*25%)= 1.13% of damage that you could potentially be missing out on.