Personally I wouldn't go 2 to 3 to 4. Either jump straight to 4 or do a rewrite. It takes someone with a good understanding of your app to assess which approach will be faster. Either way, make sure you spend some time learning Ext 4 before you start the switch. Writing a small practice app is a good way to learn and allows you to make your mistakes on a smaller scale before you write the code that matters.