Four months PO neck lift - will this resolve? These are all after pics. What's the best resolution? (Photo)

Telling you exactly what is there and the treatment is difficult without an exam . . . . but the most likely culprit is some recurrent laxity of the platysma muscle. This would be the reason for the Botox injection you received. There is another muscle (digastric) and even some other soft tissues in this area that could be leading to your fullness. In any case, the most likely long-term solution is a "tuck up" procedure which can occur in some cases 10-20% of the time depending on the surgeon. This might be amenable to reopening your neck incision and tightening/treating this area, but I agree that you should follow with your surgeon as they will know what is best based on the surgery they performed and what successes/failures they have had in their practice.
Best wishes.

I recommend seeing a plastic surgeon with at least 6 years of integrated plastic surgery training and ideally an additional year of aesthetic surgery fellowship training. Injecting Botox into the neck is usually not a solution as this will only temporary paralyze the platysma muscle in the area that was injected (the platysmal bands are truly a surgical problem). I would recommend a revision facelift/necklift to address the underlying problems with the underlying fat (asymmetry/unevenness) and tightening of the underlying muscle belly. This is my professional opinion only.

A one-sided vertical band indicates a muscle prominence likely on the platysma or underneath it. The one-sided fullness might also indicate a problem with an enlarged submandibular salivary gland. Do not try a thread. If still present in a few months, it would require revision with attention to the contents under the platysma which is not routine for most plastic surgeons. Your revision surgeon would likely require lateral platysmal pulls in the form of a neck lift. Make sure you do not get more of the same as the first surgery as it will not likely resolve the problem.