Where do the four ADPs come from in the second stage of glycolysis?
I think I understand your question, and stumbled upon it because i was wondering the same thing while studying for my Microbio test. I know its a year late, but someday someone else might need this info...so here it is. 

To put it simply: there is a second pair of phosphates added to the two 3-carbon molecules cleaved from the preparatory stage before the 4 ATP are finally formed. These get added by an enzyme while reducing the NAD+ molecules. 

In depth: The first pair of phosphates the cell uses to synthesize ATP come from the initial 2 ATP invested during the prep stage, but an additional 2 phosphates are added when the cell oxidizes the two Glyceraldehyde, 3 Phosphate (G3P) molecules to form NADH. In this step, two electrons are taken away from the G3Ps and an available inorganic phosphate is added to the "1" position, forming "1,3 Biphosphoglyceric Acid " (1,3BPG). Afterwards, the molecules each have 2 phosphates for a total of 4 phosphates available to get oxidized and used to synthesize 4 ATPs.

This step, (the addition of an inorganic phosphate as an intermediary step) is SO overlooked by regular textbooks that its not even funny. mine doesnt even list the enzyme; I had to look it up. 
but yeah, that's the source of the additional phosphates available for forming the 4 ATP molecules of glycolysis, and possibly why you didnt see it from the diagram in your textbook (you're not alone!). 

Hope that helps!

Sources, as of 4/18/17:


https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/glycolysis/a/glycolysis
https://en.wikipedia.org/wiki/1,3-Bisphosphoglyceric_acid