Simple python script

I want a simple script which will do the following: I have a csv with the following structure: 1 321 345 1 2 654 310 1 3 987 341 1 4 951 345 1 11 123 339 2 22 456 332 2 33 789 338 2 I want to end up with this (as a csv): 1 321 345 1.001 2 654 345 1.001 3 987 345 1.001 4 951 345 1.001 1 321 310 1.002 2 654 310 1.002 3 987 310 1.002 4 951 310 1.002 1 321 341 1.003 2 654 341 1.003 3 987 341 1.003 4 951 341 1.003 1 321 345 1.004 2 654 345 1.004 3 987 345 1.004 4 951 345 1.004 11 123 339 2.001 22 456 339 2.001 33 789 339 2.001 11 123 332 2.002 22 456 332 2.002 33 789 332 2.002 11 123 338 2.003 22 456 338 2.003 33 789 338 2.003 Each 'set' is defined by the fourth column. I want to repeat each 'set' with the value in the third column repeated and an incremental value added to the fourth column to give each set a new unique identifier. A simple file open and save using EASYGUI would be a plus.