The game has been updated and the error that caused the game to stop responding during day 4 has been fixed. A patch for the game has gone out to all current owners.
If you purchased the game after the update you should get the correct version automatically.
The patch also fixes a few other issues. You can see details in the patch notes / update history.
Because the fix for the error had to be implemented before day 4, auto saves of day 4 will not have the correct data to work. Other previous auto saves (of earlier days from the bugged version) may not work correctly either. This only applies to saves made with the previous version of the game. All new saves and overwrites made while playing the patched version should function correctly.
Thanks so much to those who helped me solve the problem, and thanks to everyone for your patience.