Hmm, might be interesting to stay for the Feast, though I've never seen it. Rick's three-day itinerary covers the artistic & historical highlights, by the end of that you might be tired out, and a fourth day to see more art might be pushing it. Some people love Venice enough to keep going back, others say three days is enough. If you're really into contemporary art, then maybe you could spend more time on Day 4 at both the Giardini and Arsenale (but still, it'd be impossible to see everything), chill out, walk around and just enjoy the city, especially away from the most heavily-touristed areas, though the city as a whole might be very crowded because of the Feast. After St Mark's, the Doges Palace, the Rialto, I'd say the next must-see sight from an artistic point of view would be the Tintoretto paintings at the Scuola Grande di San Rocco. And Rick's Accademia self-guided tour (in his book) is useful for a visitor with limited time. I found Venice to be rather expensive though, staying an extra day will eat into more of your budget. Or, as I've read elsewhere on the forum, plan on going back!