(Update)
You can use this logic in your procedure :
Declare
@firstname varchar(10)= 'aaa',
@lastname varchar(10)= 'ddd',
@output varchar(20)
insert into name_table
select
@firstname,
@lastname,
substring(@firstname,1,1)+
substring(@lastname,1,1)
+RIGHT('000'+cast(COALESCE(max(substring(output,3,4)),0)+1 as varchar) ,4)
from
name_table
where
output like substring(@firstname, 1, 1) + substring(@lastname, 1, 1) + '%'
select * from name_table