Baseball math

Yesterday, as we were driving home from New York, we turned on the radio to the Mets game with the Nationals. The announcer said something like this: Esteban Loaiza is pitching in an unusual situation. He's already driven in a run before taking the mound for the first time.

I immediately started trying to figure out the minimum number of runs the Nationals must have scored in the top of the first inning if Loaiza had driven in a run. Before I'd finished, the announcer reported that the score was 6-0. But six isn't the minimum number of runs. I'm pretty sure the answer is 4. I've tried to come up with an unusual way that they could have scored only 3 runs, but I don't think it's possible. Any takers?