The 4th Great Grandson is the Great Great Great Great Grandson of the individual.
Son
Father
Grandfather
Great Grandfather
Great Great Grandfather
Great Great Great Grandfather
Great Great Great Great Grandfather
You Aunt is you grandson's Great-Great Aunt. Your grandson is your Aunt's Great-Great Nephew.
Your brother is a great great uncle to your great grandson. Your great grandson is your brother's great great nephew.
Your first cousin's grandson son is your grandson's third cousin. Your second cousin's grandson is your grandson's fourth cousin. Etc., etc.
Your aunt's great great great grandson is fourth cousin to one of your parents and therefore your fourth cousin, once removed.
Your sister is your grandson's great aunt. He is her great nephew.
Your great aunt's grandson is your second cousin.
Your niece's grandson is are your great-grandnephew.
Your great aunt's great grandson is your second cousin, once removed.
Your great grandmother's brother's great grandson is your third cousin.
Your sister would be your grandson's great aunt.
Your grandson is your brother-in-law's great nephew, and your brother-in-law is your grandson's great uncle.
Your brother's grandson is your great nephew and you are his great uncle if you are a man, or great aunt if you are a woman.