well ill give u a list of what i think u will mainly need.

front susspention arms
rear susspention arms
front and rear dog bones
front and rear drive axles (not sure if the rs4 ones have the right inside diamiter )
2.2" truck wheels and tires
front bumper
heavy duty front and rear diffs ( u will need them at some point anyway )
front and rear diff axles (not sure if the rs4 ones have the right inside diamiter )


and i think thats all