If the half life of a radioactive isotope is 4000 years, how much would be left in a fossil after 4000 years

Since the half life of the radioactive isotope is 4,000 years, after 4,000 years the amount of the isotope would decrease by half. (Hence the name, half life.) So, after 4,000 years, there would be half of the isotope left in the fossil.