Generate a alphanumeric id for primary key min 4 digits

hi

How to generate an min 4 characters alphanumeric value to insert as a primary key value along with inserting  a new record. The 4 characters should be increased to 5, 6 , 7 when there is a deficit of available alphanumeric values that can be generated by the function that is generating the alphanumeric value.

I don't want to use unique identifier or autoincrement integer values

thanks