Firstly, in order to deeply comprehend the historical context of scripture, it is well and good to know the religious meanings, eg. the inspirations and purpose of the writing, and as it seemed like you were in the dark as to the meaning of 4000 bc, I thought I'd enlighten you.
As for the 2nd part of your question, regarding the big ages in Old Testament, Charles and others have covered this well, but in essence it relates to timekeeping / calendar in use during that era. Before Akhenaten / Moses / Musa (and after, in Babylon times ...), much of Middle Earth was still on the lunar calendar, so divide by twelve, eg 720 years equals 60 years etc.
Even in ancient Egypt, the solar calendar (or precise year) was guarded closely by the high priests, the public were on a "vague year" which overlapped but was not as accurate as the solar year, which was measured at the equinox (eg. Temple of Amen-Ra at Karnak / Thebes) and solstice points.
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