If you're already a fan of Suikoden you'll want to get IV just for completion's sake. If you're not already deeply into the series, and unwilling to forgive or overlook a pretty flawed game, steer well clear.
It wouldn't be the first time I bought a flawed RPG because I bought Okage: Shadow King and that game was god awful in my opinion.
Shadow King tried so hard to be funny but in the end it didn't make me laugh at all but Paper Mario 2 made me laugh my ass off.