The third number relates to Everyone (the first two are Owner and Group). 4 is read only which is why that does not work, 2 is write only and 6 is read/write (7 is read/write/execute).
In general, you should not be giving everyone write access to your server. The best approach where write access is required is to use chown
to change the owner and/or group to match the user that your webserver is running as, then use 644 or 664.
For Mac, this thread: Correct owner/group/permissions for Apache 2 site files/folders under Mac OS X? provides some pointers on how to do this securely.
If you just want to change your upload folder, try:
sudo chgrp _www /path/to/upload/folder
sudo chmod 664 /path/to/upload/folder