Since both of you are working and each claiming 4 each that is why you owe taxes. The following may help.
https://www.irs.gov/individuals/tax-withholding-estimator
and
https://www.vanderbilt.edu/studentemployment/students/W4_Guide_For_Students.pdf
Yes only one of you should report the 4 dependents on one W-4 or you you could each report two on the W-4.
Go to this IRS website and use their Tax Withholding Estimator for each of you - https://www.irs.gov/individuals/tax-withholding-estimator
Since both of you are working and each claiming 4 each that is why you owe taxes. The following may help.
https://www.irs.gov/individuals/tax-withholding-estimator
and
https://www.vanderbilt.edu/studentemployment/students/W4_Guide_For_Students.pdf