Is it possible to open or import Mathcad Express 4.0 file in Mathcad 3.0 Prime (full academic license)?
Not by any method anyone has discovered.
See the answer here: Express 4.0 file to MathCad 3.0 Prime - PTC Community
I have full versions of 3.1 and 4.
You can cut and paste line by line.
I couldn't cut and paste multiple lines at once though.