Four feet is too long for one trap...the trap arm would collect crud and be open to the air and eventually generate some nasty smells. Depending on where the vent is, you might need some fancy footwork if it is essentially an island (you can't go up with the vent). You'll probably need a loop vent. A quick search will show you some possible ways to do this. A diagram of what you have and what you want might make it easier to comment.