[Nimbus]

It depends on what you expect to do in each park. If you only plan to go on 1 or 2 rides, and only spend 2 hours or less, in each park then doing 4 parks in a day is doable. If you want to do most of the attractions in each park then stick to one park per day (even then, one day may not be enough - we often visit Epcot on two days, for example).

If I was you I'd stick to doing 2 parks in one day.

BTW, you can walk from Epcot to Hollywood Studios via the International Gateway between the UK and France pavilions. Or get the boat if you want to save your legs.