4yo daughter potty trained for over a yr was constipated, one dose of miralax (polyethylene glycol) cleared it up but now has loose stool accidents&afraid to go on potty?

Constipation. It may not be "cleared up". Chronic constipation leads to rectal vault distending. This in turn leads to encoresis or stool soiling. It necessitates weeks to months of regular bms to reverse the distention. Fiber and laxatives help. See a pediatric gastroenterologist.