CSV Manipulation in powershell

Hi there, I am not strong with powershell I have essentially just been pickup up things as I go. I have been asked to perform a task at work and I think it is probably possible but my skill are lacking in this area. I am trying to make a script that will open a CSV and grab the last 4 charaters from one value and append it to another. I can import the CSV with headings 1..22 and need get data from 18 and append to the end of 15. If someone can point me in the right direction that would be much appreciated.