Ok, I'm also known, in some circles, as 'Mr. Hypothesis' so here you go.
First of all. 4mg/ ml is very small. it's like, one part in 250, so even if the dosages were way way off it'd not really change the viscosity.
But, from the taste, it's clear they put other stuff in there. Stuff like B-alcohol, or glycerin.
And that's it. A little less glycerin (or prop glycol or whatever) and it's thinner.