Is there anyway to add a unique sequence number based on a given format.

For example

I input the 1st letter of the first name, first 4 of last name, month, and job number

Tom Jones, Month is January, 1st job

t_jone_01_01

next job would be

t_jone_01_02

Is there any code that would automatically increase the last two digits by one each time.....03,04....10,11....

Thanks for any help

Josh