Transiting Through MAD - 2
"European flyer" was wrong to state that the inter-terminal bus is "the only option" (post 4).
The metro connects T4 with T1-2-3 and even though it stops at Barajas first (and the walk from the station to the easyJet check in desks isn't that short) it is another option.
Another option is to take a taxi (leaflets giving the approximate fare for the trip between terminals are available at the airport).