The costliest merge

Merge sort's merge step combines two already-sorted lists by repeatedly comparing their front elements, emitting the smaller, and - once one list is exhausted - copying the remainder of the other with no further comparisons. You must merge two sorted lists of four elements each (eight distinct values total). Over all possible ways the eight values could be split between the two lists, what is the maximum number of front-to-front comparisons a single merge can make?