Melt butter in a medium saucepan over medium-low heat. Whisk the flour into the butter and cook 2 minutes. Reduce heat to low and whisk in 3 cups chicken broth. Whisk cornstarch with remaining 1 cup chicken broth then whisk into the skillet followed by chicken bouillon
and all seasonings. Bring to a simmer, whisking constantly, and continue to simmer until thickened to the consistency of thick gravy (it will thin in the crockpot). Pour over chicken in crockpot.