Hi Zimmy,
By "four into one", do you mean four separate header pipies going into four pipes in a square formation in the sidepipes?
If so, mine was like that and it was never really a problem.
I don't think you'll ever get it really air tight, but it never leaked so much as to be an issue. In fact a little fresh air being drawn in adds to the drama on down-changes ...more noise and big flames
I welded a pair of tabs on the top two headers and collectors that bolted together and held the collectors on....does that make sense ? It's quite difficult to describe without a picture.
Cheers
Paul