That script won't work on SQL Server 2000. You did post on a SQL Server 2005 forum after all.

Run sp_who2, find if the BACKUP processes are being blocked, if so, by which process.

Do note that given the backup command you are using i.e.

BACKUP DATABASE 'Databasename' TO DISK = 'E:\'filename'.BAK'

you are appending backup sets to the file, so after 4 days of backing up the 4 databases totaling 108 GB, and assuming most of the database space is used, you will run out of space on that external drive. Also, do check if the external drive has been formatted using FAT32 or NTFS.