HT falling off is the same as disembowel falling off, its not a big deal.

Your main combo damage average numbers are like 250 something, so maybe 275 w/ HT, but a 4th combo w/o HT is going to be at least 400 or so, bringing your average dps way up.

Furthermore even one tick of missing phleb doesn't matter, it'll definitely cover it.

It may eat away at the damage bonus of course, but theres no way (even if it was the same potency as full thrust) that you'd actually lose dps from a 4th combo.