What "Junior" and "Senior" Really Meant
Terms like Junior (Jr) and Senior (Sr) were used in relative (bad pun) fashion in colonial America and the early United States, and just meant two people of the same name lived in proximity and one was older and one was younger. While it often meant father and son, the two individuals could be more distantly related (I have seen second cousins use Jr and Sr), or not related at all. I have also seen Jr and Sr used with females.
A variant of Jr and Sr were the terms "the Younger" and "the Elder", which are often used in old records.
When there were more than two people of the same name in an area, record keepers became creative; for example, my ancestor Peter Troutman was called Peter Troutman the Blacksmith to differentiate from the other Peter Troutmans in Cabarrus and Rowan Counties, NC.
Also, a person could be called Jr as a young man (usually to differentiate from his father), but then be called Sr later in life (usually to differentiate from his son).
Roman numerals after names like III and IV are purely an invention of genealogists. In over 25 years of looking at old records, I have never seen Roman numerals after a person's name, other than royalty. For example, in Prince Edward Co, VA chancery case 1762-006, Margaret Womack vs Richard Womack, Sr [mistakenly listed as "Margaret Womack Wilson", a misreading of "Margaret Womack Widow"], Margaret (Archdeacon) Womack's deceased husband is called Richard Womack Jr, and her father-in-law is called Richard Womack Sr, though genealogists refer to them respectively as Richard Womack IV and Richard Womack III.