Delete the 4 buttons from the form.
From the Toolbox (View menu), place an Option Group on your form.
Put 4 options buttons inside the group.
Now that they are in an option group, the user can select one only.
To examine which one is selected, look at the value of the group. If you are
trying to store this into a field in your table, the field should be of type
Number, and it will store the value of the selected button.
If you see two selecting at once, it means the two buttons both have the
same value. This can happen if you duplicate a button in the group instead
of adding another one. You can solve the issue by setting the Value property
of the button (Data tab of Properties box).