Is 4 months old too young for a door bouncer?

That's about when my daughter started using hers. The baby doesn't need to be able to sit up independently in order to use one, as long as he has relatively good head control he should be fine. I wouldn't leave him in there too long, it's not good for the hips but 10-15 minutes won't do any harm. Make sure the crown molding is at LEAST 1/2inch thick and stable. Those door jumpers always made me paranoid.