A few differences between 4 and 9:
9's try to sugarcoat things to avoid conflict.
4's ultimately don't care as long as they express their unique views on things.
4's primary mode during depression is to take their emotions into their identity... They're more willing to look into the dark side of themselves and be completely honest. Although, once a 4 looks too much, they might grab onto a dark identity and make it into their character.
9's look to get away from their depression through influencing their emotions. This does not mean that 9's are unaware of their darker emotions, they just don't want to identify with them too much for fear of permanently destroying their inner peace.
4's care so much about individualism it really hurts to see some of them... 9's would rather keep the peace than express their negative side for all the world to see.
Also, huge difference between Fours and Nines you see on this forum all the time. Fours express their views passionately and without shame, and Nines try to be a little bit more tact about it, sometimes not even taking a side.