How to define a MLP with multiple outputs
Have four neurons in your output layer, depicting gain $A, B, C, D$ and apply relu activation function throughout all your layers since it sets all negative values to $0$ and passes the rest. 

Also you can use a simple mean squared error as your loss function. This setup will allow you to predict non-negative values, greater than $0$ for your gains, which is, what I believe, you require.