Hi,
First off, there's no need to stay near Milano Centrale Station. Public transport is excellent and you'll be able to go there to catch the train Venice very easily from any hotel that is more centrally located. If you look at a map of Milan, you'll see a first and a second circle of boulevards. I would prefer to stay within the first circle, or the second one at worst (near Corso Magenta / Cadorna station is quite convenient in that case), that way you can walk most places in town.
For airport, if you have the choice then LIN is much more convenient as it is closer to downtown. But most international flights go to MXP, and the difference is not worth a connection. If you land in MXP, cabs are very expensive, so take the Malpensa Express train to town (to either Cadorna or Centrale Station).
Regarding sights, most folks here would consider that 4 days is a lot for Milan. Indeed, once you'll have visited the Duomo, strolled around the center, visited the Pinacoteca Brera museum, Santa Maria delle Grazie (bookings needed) and enjoyed Aperitivo drinks and food by the Navigili canals, you'll have experienced the highlights.
But you can make day trips: Como, Bergamo and the Certosa di Pavia are all worthwhile day trip destinations (or half day for the latter), easy to reach by train from one of Milan's many regional train stations.