This can be good if you are trying to mute the 4th string. You can use one or all of your fingers to pick the strings. I would recommend not using your thumb to pick. You can and if you do just be aware that, you lose speed going from the thumb to your other fingers and the thumb sounds different from all the other fingers. So unless you are playing slow and only using your thumb, use your other fingers to pick.
Most people feel comfortable playing with the fore finger and middle finger.
Otherwise know as the 1st and 2nd fingers. It may feel right for you. Otherwise don't worry if you cant. You can play pretty darn fast with only two fingers. When you pick the strings you want to kind of rake them.