April has fewer tourist arrivals than during May, however be aware that the last week in April is one of the busiest of the year because of the presence of two national holidays (April 25= Liberation Day, May 1= Labor Day). May has more foreign tourists, but fewer Italian tourists because school trips generally don't take place in the last month of the school year. The month of April has tons of middle and high school kids on school trips (which I find rather obnoxious inside museums). That applies to Venice, Florence and Rome, which are the most popular destinations for school trips.
Flights back to North America leave in the morning. If your flight stops in some European hub first, coming back from Venice will entail a very early wake up, because the flight back from VCE will leave before 7am (and you need about 1 hour to get to the airport from Venice) to connect to the flight across the Atlantic. That is why I would suggest to return from Rome, which is close to the airport (30 min).
VCE has however a couple of non stop flights to the US with Delta, United, US Air (to JFK/ATL, EWR, PHL respectively), in case you are interested. Those flights depart a bit later in the morning, therefore if departing from VCE, they would be at a more decent time. Be aware that only the Delta flight to JFK is year round, the others are seasonal and operate only in summer.
Also, and this is my opinion only, if in Tuscany you intend to visit both Florence and the famous Tuscan towns, you may want to borrow one night from Rome. Four nights in Tuscany are only 3 full days and with that you can only visit Florence (2 days) and maybe Siena (1 day). If both Florence and Tuscan hill towns are part of your itinerary, I would do: Venice 3 nights, Florence+Tuscany 5 nights, Rome 4 nights. With 5 nights in Rome I would definitely find the time for a day trip somewhere nearby (e.g. Orvieto).