I lean towards 5,4 being the intended correct answer but I hope the markers will be generous in view of the ambiguity.
The title of the puzzle is fives not five suggesting to me that both sets are important. If only X needed highlighting then it is feasible that people could miss seeing Y completely (as I did initially) and still submit a correct grid.
More often than not the preamble will indicate the number of cells to be highlighted and that would have been helpful here. I wonder if this information has been omitted in order to let people fall into the trap of missing the second group.
Lastly I don’t think more than means rather than in normal use despite the second adverb definition in the BRB.