Sicilian
Queen's Gambit
Ruy Lopez
French Defense
I've been thinking that I might prefer to substitute the Nimzo-Indian for the Queen's Gambit. I think you can make a case for including either one, but not both, since they're pretty similar and actually can transpose.
Why the Queen's Gambit? Perhaps to learn how to respond to 1.d4?
the problem(well not problem more of a setback or drawback) with e4 would be there are a couple of pretty vexing defences against e4 against d4 the positions are more or less somewhat similar though some can be quite different like the grunfeld and the QGD very different.