As these are two very different cities it's a rather odd question. One is flooded with tourists, has alot of temples a big market in the evening, two rivers and loads of mediocre (at best) restaurants. The other is a laid back city with low key eating establishments, not much to do in town but a bounty of easily seen, with transportation, out-of-town sights.

With only 4 days don't try to split time between the two as there's no direct way to get from one to the other.

QV has a flight LPQ-CNX but you'll spend an entire day getting between LPQ and CEI.