KO4 is the turbo off the bigger powered 1.8s 210bhp -240 bhp standard and its the same ones on the S4.
not a direct bolt on in most cases.
you could look for a K03s turbo from the 180 engines (think audi had them at 190 due to the extra losses through the quattro) this is a mid way between the two, it has half the internals off the K03 and half of the K04, not exactly a direct bolt on as the coolant and oil lines are different however it should fit the exhaust manifold and downpipe where as the K04 will require the bigger manifold and a fabricated downpipe
just for info if you go into VW Audi etc the K03s doesnt exist supposidly its just the K03 with different part numbers