Forms :: Create A Unique Customer ID

So I have this field in a form/table called "CustomerID". This is to be an ID number for our customers that includes the first four letters of their last name (LastName field) followed by four digits.

This ID is created when a button is pressed in the form. After that, code needs to grab the first four letters of the last name (LastName field in the form and table) and then go to the Contacts table and see if there is already a CustomerID that starts with those four letters. If there is not, then the CustomerID will be those four letters followed by "0001". If there is, it needs to be those four letter followed by the next number sequence that still does not exist for those four letters.

For example:
First four letters of last name = zabo
Check to see if those four letters exist in CustomerID returns negative then
CustomerID assigned = zabo0001

Another example:
First four letters of last name = zabo
Check to see if those four letters exist in CustomerID returns zabo0001 and zabo002 then
CustomerID assigned = zabo0003