Delete folders in shell script after so many backups

Hi,

I have a backup script which copies the home dir to an external hard drive every 6 hours.
What i want to know is if there is a command i can put at the end of my backup script which will only keep the last 4 backups. All the others can be deleted.

My dir structure is:

-Root
--Day
---Daily Backup
--Week
---Weekly Backup
--Month
---Monthly Backup
--6 Hour Backup1
--6 Hour Backup2
--6 Hour Backup3
--6 Hour Backup4
--6 Hour Backup5
--6 Hour Backup6
e.t.c

So this means i cant just delete all the directories only the ones which follow the pattern of the 6 hour backups (regexp?)

Any ideas?

Thanks in advance