You'd think so, but it's not the case. B4, with the extra F4s included (e.g. you spend 8.4s seconds for a total 1196 potency) is worse in terms of potency per second than every other spell in our single target rotation except Fire 1. Now, it's still better than the REST of our spells - regaining mana with B3, T3, B4, F3 is superior to B3, T3, B1, F3 - but it is still bad compared to our powerhouses.
If you only ever needed to cast one spell between F3 and B3 you'd never want to use B4, and the math bears this out. Unfortunately I'm stuck posting on my phone and can't type out all the numbers easily (for some reason I can log into the lodestone on my PC but not the lodestone forums, something about a session key).
If Square wants us to B4 every cycle, they need to dramatically improve umbral hearts somehow. My idea is UI-style doubled cast speed along with the existing mana discount.
