I always type INTP. What changes is my enneagram. In the Risso-Hudson test, which admits a max of 32 points for each enneatype, I score 29 for 5, 27 for 4 and 23 for 1 (the rest under 15, 2 with 0 points). When I feel depressed, e4 takes the first place. I mention the scores because when I feel fine the difference is not so much.
The difference "in action" is that when I feel five (fine), I tend to be more curious and more assertive regarding my (random) goals, and when I feel more four is when I sense I feel somewhat attacked by life, often summarized in the phrase "why does this happen to ME?"
Anyway, that's a slight difference, and I agree that I'm always a healthy/unhealthy 5 (I tend to withdraw and hoard things and pieces of knowledge until I feel I have what I need to go face the real world again). I'm always a healthy/unhealthy INTP (in my worst times I'm untraceable and let everything fall apart, including health).
I also think it depends on how mature you are and/or the needs you feel you have. In my early teens, when I felt sad I'd probably turn very Fe-ish. Later, when I felt like I was alone in the world to an extent that even a 100% introverted and shy person like me would find exasperating, I'd turn everything to 11, everything in excess: daydreaming, eating (or not eating), forcing myself to do things I later regretted intensely, etc etc etc. And now when I feel bad, I just withdraw from everything and everyone and, except for Fe (because I turn REALLY cold -more than I already am-), I feel my other 3 functions are enhanced: excessive analysis, excessive introversion and shyness, complete lack of control regarding organization, timetables.. very P-ish.