As long as they have identical specs, it should make no difference. Run memtest86+ for several hours to make sure there are no memory errors.
I doubt they are the same spec's Chris, as the 2 1gb units are about 3 years old and the 2x2gb units are brand new and I believe Kingston have upgraded their product.. I suggest they be treated as individual pairs.
I would have thought that the 2x2 should act as the frontmen and the 2 x 1 units secondary?? That is just my thinking on the subject.