To be honest I wouldn't contemplate doing that kind of work on a 64bit machine with anything less than 8Gb of RAM.
"If I decide to upgrade the RAM later on, is it possible to do this with a Macbook Pro or Air?"
Macbook Pro - yes
Macbook Air - short answer is no, but it is possible. My advice, given your work requirements, is to stick with a Mac book Pro and try to go for at least 8Gb RAM.