If a spinner was picked instead of a 4th fast bowler, I think it should be Smith. Even though he isn't as 'good' as Hauritz, his batting would be very useful. I don't think the Gabba will require a spearhead spinner, so the fact that Smith isn't that good shouldn't even matter. Most of the wickets should come from the fast bowlers anyway.
So my preference would be 4 fast bowlers (not including Watson), followed by 3 fast bowlers and Smith.