The following four generations concern four stages of a man in his first (mortal) age:
Proverbs 30:11-14 KJV
11 There is a generation [1] that curseth their father, and doth not bless their mother.
12 There is a generation [2] that are pure in their own eyes, and yet is not washed from their filthiness.
13 There is a generation, [3] O how lofty are their eyes! and their eyelids are lifted up.
14 There is a generation, [4] whose teeth are as swords, and their jaw teeth as knives, [Dan 7:7 - the fourth-generation "beast" - "great teeth of iron"] to devour the poor from off the earth, and the needy from among men.
Moreover the Master confirms this understanding in multiple places:
Matthew 12:39-45 KJV
39 But he answered and said unto them, An evil and adulterous generation [1] seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas:
40 For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth.
41 The men of Nineveh shall rise in judgment with this generation, [2] and shall condemn it: because they repented at the preaching of Jonas; and, behold, a greater than Jonas is here.
42 The queen of the south shall rise up in the judgment with this generation, [3] and shall condemn it: for she came from the uttermost parts of the earth to hear the wisdom of Solomon; and, behold, a greater than Solomon is here.
43 When the unclean spirit is gone out of a man, he walketh through dry places, seeking rest, and findeth none.
44 Then he saith, I will return into my house from whence I came out; and when he is come, he findeth it empty, swept, and garnished.
45 Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. Even so shall it be also unto this wicked generation [4].
This speaks from the beginning: Qain and his line, just as it was laid out for you in one of the posts requoted above herein, (titled "The Book of the Dead"). Qain is the prototype for the goat to-for Azazel with all the sins upon his mortally wounded head, sent forth into the dry-arid places of the desert-wilderness, that is, "the land of Nod" or wandering. But Moses warns, "be sure your sin will find you out", (Num 32:23, and that is why the goat sent away is not destroyed in the text, even though it did become a tradition to send it over a rocky-jagged precipice). So Qain comes back to his "house", (YOU, and there are no exceptions to the rule given by the Master), and he finds the house empty, swept, and garnished, (with new idols), and he goes and gets seven other spirits more wicked than himself, (the line of Qain as shown above herein). And the seventh, Tobal-Qain, brings forth Qain with pomp and fanfare: thus Qain is the eighth and of the seven, for he was the first which was sent away into the desert to begin with.
With this supernal understanding held first and foremost it is not difficult to see that there was plenty of time for many other people to have been born of the physical seed line of Adam whether or not "Adam" is intended as a single man or a federal head representing all of humanity.
Four Generations:
1) Adam
2) Qain
3) Abel
4) Seth
New Man ~ Enosh-Man
And Enosh is he that began to herald-preach-call out in the name of Yah the Elohim, (Gen 4:26). Thus, with the above supernal truths being held first and foremost, there is plenty of time in the first four generations for many other people to have been born of the physical seed line, but the point not to be missed should be this: the holy seed line of Adam to Noah, (and all the way through the entire record of the scripture), is not physical, but SPIRIT, because it is by faith and the hearing of the preaching of the truth beginning with Enosh. For the same reason Peter calls Noah "the eighth Preacher of Righteousness", (2Pet 2:5), for Noah is the eighth from Enosh, the first Preacher of Righteousness according to the scripture, (Gen 4:26).
Preachers of Righteousness:
1) Enosh - Gen 4:26
2) Qainan
3) Mahalaleel
4) Yared
5) Enoch the Prophet
6) Methuselah
7) Lamech
8) Noah - 2 Pet 2:5