the filters usally come with a 4 inch opening and a 3 inch adapter to make it fit either size, the pvc pipe i belive was 3 inch pipe, but the one side is 4 inches because its fatter to let a 3 inch pipe slide into it and be sealed in its original use it was intended for.

note how one side is fatter, thats the 4 inch side, the thin side matches the t body.this is how i painted mine too.