Hmm..... Two different conversion programs I use say different.
Your conversions were correct the first time. However, you are missing some units. You are talking about torque, which is force x distance. Servos are rated in oz-in, not oz. To know what the 4-6kg rating converts to in Imperial units, you must also know the moment arm measurement being used. If it is also 1 inch, then your original numbers are correct. If it is different, then you need to convert for the arm length as well.
Mark