I think Dragon Quest V is better than Dragon Quest IV. The story in DQ5 is a lot more endearing as are the characters. The game does a great job of taking you through the main character's life. My only complaint is that you don't get much freedom to go where or do what you like early on. Of course that changes later. Monster catching in DQ5 is also pretty cool as well. Having a team of the MC, a metal slime, a regular slime, and a slime knight is neat.
Dragon Quest IV was pretty neat with its "chapters" but while I kept wanting to get back to the main character during the first few, the game kept making me play as character after character that I wasn't always interested in. I also didn't like the final boss. The motivation of the boss just wasn't developed enough so the end felt kind of anticlimactic. Oh yeah, and Dragon Quest IV was fairly short. I beat it in 11 hours. There was only one part of the game where I was confused about where to go. For most of it, the game basically told you where to go next or for one reason or another it was obvious.