I bought and installed a catless downpipes for my X3 from Ultimate Racing about a year now and it's been working great.
http://www.ultimate-racing.com/colle...3-35i-downpipe
Seeing as the X3/X4 35i both share the same downpipe part number it should be able to fit your X4.
On a side note, since I've gone catless I hear my wastegate rattling like crazy. It affects some N55 owners but I don't think it hampers the performance. Just something to keep in mind.