Runner knocks 1st baseman down

[Steve M]

As has been said, 4 feet is at most one running stride. I don't see that as enough time for the runner to avoid or slide. And it does sound like the throw brought F3 into the runner's way. F3 had the ball, so no obstruction. The runner did not alter her path to go into F3, so I probably have just a wreck. From your description, the wreck is probably front-to-front and I'd expect there was contact with the gloved ball before the actual wreck (just before, but still before) - there's your out.