That is a reasonable dose, and on the high-end, given the 0.05 mg/kg - 5.0 mg/kg recommended range.
At your vet's recommended dose, this means a 42 lb dog would get 76.4 mg.
42 lb = 19.09 kg
19.09 kg x 4 mg/kg = 76.4 mg
If you had a 2 mg/ml bottle of dex, this would mean your dog would get 38.2 ml of dex for shock.
With a 4 mg/ml bottle of dex, your dog would get 19.1 ml for shock.
Jack