I've never done it on a Subaru before, but I have on a Ford Fairlane V8 and a Mazda RX7.

Water pumps are fairly straight forward, if your mechanically minded. It can also be a VERY messy job.

The bits I'd be concerned about are the pulley removal (you might need a puller for that), and obviously stuffing up the cam timing..

I cant really see it happening in 4 hours if you've never done it before, but you never know

Do consult someone who has done it in the past, thats my advice.