Greetings!
The giants in Genesis 6:4 are not just ordinary humans. They are not just ordinary humans as their breed is made of the breed of the sons of God and the daughters of men, which simply means they are breeds from two different beings.
These giants are the opposite of what science takes giants to be. Science always takes giants in human form to be a being of excessive growth hormones being produced by the pituitary glands, which might come out with defects and weaknesses in their body structure.
However, in the case of these ones, they are strong and mighty giants which were of old and men of renown which means, there's no effect of excessive growth hormones as produced by the pituitary glands in them.
Genesis 6:1-5 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be a hundred and twenty years.
There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bore children to them, the same became mighty men which were of old, men of renown.
And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
As we can see in the above scriptural verse, these giants like I said before are not just ordinary humans. They are the result of the crossbreeding between the sons of God who are angels and the daughters of men.
Job 38:4-7 Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding.
Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it?
Whereupon are the foundations thereof fastened? or who laid the corner stone thereof;
When the morning stars sang together, and all the sons of God shouted for joy?
In the above scriptural verse, we were made to know that the morning stars and the sons of God as used here are Angels and that they have existed even when humans have not come to being. They were there in the process of the creation of the world with God, but when they saw the daughters of men, they fell in love and wanted inter-marriage with them.
Thanks for reading.