Haiku supports gcc4 too. You can download & use gcc4 hybrid - this is a gcc4 compiled version of Haiku with gcc2 libraries for running BeOS software. You can also compile with gcc4 on Haiku. Many games recently listed on Haikuware are gcc4 compiled. Like, Heroes Of Might & Magic II. Also, other ports, like Qt, KDE, etc. compiled with gcc4.
Just to note, Official Haiku releases are gcc2 hybrid though. This may or may not change for R1. Have to wait & see.