The Type 4 is fun because of its derpiness, but it isn't exactly what I would call a great tier 9 heavy - especially in a field of very good tier 9 heavies. However, it is great at a few things when using the 15cm:
First off, it is the anti-Maus - or fill in any other super heavy you want there. By virtue of the fact that it can do 400-600 damage per shot when hitting armor and more when using other tactics (see below), it is something no heavily armored tank wants to run up against. It is good at this, not only because it does damage when it hits an armored opponent, but because you can splash and damage lightly armored areas for even more damage.
1) Hitting the Maus on the underside of its turret will splash down on its lightly armored engine deck. Not all tanks have the layout of the Maus, but many have things you can hit that will cause splash into more vulnerable areas.
2) When someone is sidescraping, you can splash off the tracks, or just next to the tracks. Not only do you do the splash damage in that 400-600 range, but you also tend to track them. Time this correctly, and you can do so before they can back up enough to get their gun on you, thus spoiling their shot and giving you time to get back into cover.
3) When going against more heavily armored foes from the front where you will tend to do less damage, you can shoot underneath the tank and splash up into the very lightly armored underbelly.