How do you compute a generation Jay?
Not using the modern understanding of a generation which is usually to be a descendant generation.
The Hebrew root word is H:1755 has the following meaning: -
OT:1755 dowr (dore); or (shortened) dor (dore); from OT:1752; properly, a revolution of time, i.e. an age or generation; also a dwelling: – KJV - age, X evermore, generation, [n-] ever, posterity.
OT:1752 duwr (dure); a primitive root; properly, to gyrate (or move in a circle), i.e. to remain: – KJV - dwell.
And also from The Online Bible Thayer's Greek Lexicon and Brown Driver & Briggs Hebrew Lexicon, we have
OT:1755 dowr or (shortened) dor : – a period, a generation, a habitation, a dwelling
- a period, an age, a generation (a period of time)
- a generation (those living during a period)
- a generation (characterized by a quality, a condition, a class of men)
- a dwelling-place, a habitation

Please notice that we have two separate Hebrew Roots words which have been translated as "generation." OT 8435 is a reference to descendant generations or Genealogy whereas OT:1755 is a reference to a period of time.
Now, in Genesis 15:16 we have OT:1755 used in this verse where it is referring to a period of time. The duration of "Aday of the Lord," "a generation" and "an age" all have a duration of 1,024 years, including solar years. So, in the "fourth Generation/Age the prophecy is referring to a period of time 3,072 to 4,096 years from the time that Isaac was born in the year 2052 BC. We know that 4,000 years after the birth of Isaac that some of Abraham's descendants returned to the Land of Canaan in their own strength, i.e. by acting God Like and not with the help of God, Himself, in 1948 which gives us a time period of exactly 4,000 years from Isaac's birth to when some of Abraham's descendant returned.
The second part of Genesis 15:16 tells us that at this time that the iniquities of the Amorite people will not yet be completed. What is their iniquities? Their claim that Jesus is not the Son of God.
God's gathering of Israel to Himself will not begin for a further 96 years after 1948 AD when He will begin redeeming Israel and plant them in His fertile soil/field.
I trust that this will help you to understand the difference.
Shalom