How is it possible that only $40 in taxes was taken out on over $4,000.00 earned income?

[pk]

it is possible -- based on your filing status  and AGI , it is quite possible that the marginal income of $4000  produced only $40 total tax before credits.  Need more details to answer the question

[KaseyO]

Last year tax withholdings were lowered, so you may have seen more money in your paycheck throughout the year in 2018. This would reflect a lower amount being withheld.