According to GameTap, the 4100 Error plaguing over a dozen titles, "Occurs with games that were packaged with a progressive download technology that is no longer in use when packaging new games. We now use a more effective and recent technology for all our new games. We have also started during the summer to upgrade our back catalog with the new technology."
The update at the end of October will allegedly be more reliable and the company expects it to eliminate the 4100 error. Regarding concerns that the errors were a sign that the service was ending, the representative stated plainly, "We are not ceasing our operations in the US."
[Thanks, Kyle B.]