How do you have to make a fourth pipe which isn't glitchy?

The only way I know is to replace the (mini-)solid-steel-pipe things... (you know ) with graphics of a new pipe.
Maybe there's a better one but I don't know.
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Right. So you mean... what?

He means the grey 2×2 tiles square that make the corner of some pipes, aka:
It is represented by those files in the polygon_unit folder:
I_do_kanagu_ld.nsbtx
I_do_kanagu_lu.nsbtx
I_do_kanagu_rd.nsbtx
I_do_kanagu_ru.nsbtx

In fact I did that in my hack, but on top of that I modified 8 other sub-files inside I_dokan.nsbtx:
do_kousa_t_ld
do_kousa_t_lu
do_kousa_t_rd
do_kousa_t_ru
do_kousa_y_ld
do_kousa_y_lu
do_kousa_y_rd
do_kousa_y_ru

Keep in mind that I sacrificed those two joint-pipes in the process:

I also tried to use these tiles for a blue pipe but I can say because of the additional colours the quality of the pipes really decrease. So I wouldn't edit these if you want to make high-quality pipes.
_________________________
Super Mario Bros. - The New Worlds | Full release
Super Mario Castle 2 | Full release
Super Mario Castle 3 | Full release

My Youtube channel
Best friends forever

Yeah true, but consider that the native NSMB pipes work with only 32 colors.
Even if you are limited to 128 colors instead of the 256 available, the overall quality should still be improved by at least 4 times if we consider that the colors are applied on the same artwork.

I'm under assumption you say the quality decreases because you try to import a NSMBWii pipe, right?
I did that for sure, but really we should just make our own pipes to get the best results.