You get this quest after completing The Defias Brotherhood III. Speak to Mathias Shaw in the Barracks of the Old Town of Stormwind. He will tell you:

The Stonemasons' Guild was run by a man named Edwin VanCleef.  VanCleef was responsible for rebuilding Stormwind after the orcs razed it in the First War.  Apparently, VanCleef and his men were unhappy with their treatment by the King after the reconstruction was complete.  That just might explain a thing or two.

I have written a more detailed account for your Master in Westfall.  Take this to him at once!

Travel to Westfall
Speak to Gryan Stoutmantle

Back to Sentinel Hill you go, but this time you can stay here for the next parts. Speak to Gryan Stoutmantle outside the sentinel tower, he says:

Did Master Shaw shed any light on things?

Edwin VanCleef?I know the name well. To think that a man so industrious and talented could turn into such a scoundrel unnerves me. I?ll need further proof before I believe this.

And this is exactly what you?re going to get in the next part, The Defias Brotherhood V.