The giants cited in Genesis 6:4 were not normal humans.
The scripture highlights that they were birthed from the union of sons of God and the daughters of men.
These giants had their reputation and were described as mighty and strong.
This reveals that they had a unique nature which differed from that of humans.
It is recorded in Genesis that during the period of raise in humans on earth, the Sons of God took the daughters of men as wives and they had children .
These children turned out as giants.
Angels who are spiritual beings were the Sons of God mentioned in this passage.
Man is flesh and this is the reason why God said that His spirit would not always strive with man.
In summary, these giants came from the union of spirit beings and humans, this explain their unique nature and strength.