A good friend of mine is getting married to a girl he has dated for four months and I'm going to attempt to dissuade him.
What's your reason for wanting to dissuade him from getting married?
I think that is a bad decision and I am against it
It's not your call to judge his relationship and what your friend should to do. If you stick your nose into his business, especially wedding plans, you might lose your friend over it.
I don't care and I still think it's a bad decision.
It very well might be a bad decision, but it's his decision to make and it would be rude of you to interfere. You should be supportive of your friend's marriage and if ends up not working out then you should support him then, too.