The 04 STI had the same bolt pattern as all other Legacy and Impreza models, save for the 05+ STI. It should fit, but may be a bit wide in tire and wheel. I'm sure that with some creative fender rolling, it'd work fine.
Fender rolling is when the lip on the inside of the fender is rolled upwards, so that it resembles a tight V or U insteald of and L, as seen from the side. This can give a bit more clearance for wide wheel/tire combinations.
Rota makes a lot of wheels that fit Subarus and SubyDude sells a lot of Rotas.
Also you're looking at over a grand on wheels if you want to go with the BBSes. I recall seeing sets of just wheels for like $800 and like $1200 with good tires.
You can get 02-05 16x6.5 WRX wheels with good tires for like $300. $200 with new RE92s.
Actually the 17x7 06 WRX and LGT wheels would probably be a better fit and are also less.
Hey I found just what I was looking for!
BBSes are THAT much!? I asked my dad about fender rolling and the first thing he said was no. Then read my mind about the STI wheels.
~Spencer
94 Legacy Turbo (550 Robtune/ej20h v2 Sti RA drivetrain)
94 Legacy Ti Wagon (5mt ej22e)
91 rhd Legacy GT Wagon (factory 5mt, ej20g)
93 rhd Legacy GT type S2 Sedan (4eat, ej20g)
91 rhd Legacy Ti Type S 1.8
03 Lincoln LS V8 Sport
08 300 SRT8