I can make a few suggestions to on things to check which could cause the problem as you have described it.
1) check /etc/fstab. is /dev/sda4 defined there with the mount point /mnt/zip?
2) what are the mount parameters (user, ro, rw, default, etc)?
3) does the mount point /mnt/zip exist? meaning, is there a sup-directory in /mnt which is named zip? if not create it (cd to /mnt, then issue the command 'mkdir zip' to create the mount point).