input file:
column1,column2,column3,jack.jack@my.com,column5
column1,column2,column3,tom.tom@my.com,column5
column1,column2,column3,mike.mike@my.com,column5
column1,column2,column3,albert.albert@my.com,column5

output file:
column1,column2,column3,first.lastname_1@my.com,column5
column1,column2,column3,first.lastname_2@my.com,column5
column1,column2,column3,first.lastname_3@my.com,column5
column1,column2,column3,first.lastname_4@my.com,column5