Just done that myself, as I'm heading into the B-spec endurance grind in GT5 and won't be able to use my PS3 for long periods.
Bought a second-hand slimline PS2 today with 1 year w'ty from a local shop for A$69, and second-hand copy of GT4 from the same place for A$9.95. So the combination cost about the same as GT5 alone, which I got on an opening special!
Looks and feels crude compared to GT5, but still good fun and has some nice tracks that GT5 lacks.