How can inconsistencies in these sodium chloride solutions be explained?
example, 4 g/L $\ce{NaCl}$ is coming out as 0.68 M/L...hopefully I'm missing something very basic here


Yes, you are off by a factor of 10. 

4 g/L $\ce{NaCl}$ is 0.068M (0.068 moles per liter of solution).

As far as the journal article, I would used the 4 grams per liter value (and other grams per liter values) and convert to moles per liter properly and not rely on the 0.08 (small capital "M" l-1) value. 

If the value is critical to your research, email the corresponding author of the paper and and ask about it in a polite way.