Note also that the poll is of "registered" voters -- not likely ones. That fact obscures even more good news for the GOP. "Likely" voter numbers register intensity, which often (though not always) is on the side of those opposed to the administration of power.
So chances are that the real margin between the GOP and the Dems exceeds even the four point edge Gallup has picked up.
That being said, of course, there's no time for complacency or for counting chickens before they've hatched. There's plenty of opportunity for the tide to shift, or for "independent" congressional candidates to split the anti-Democrat vote. It's part of the Republican party's job to make sure they're offering a clear and attractive alternative to the big government, freedom-sucking agenda being imposed on America by the Democrats.