I switched to the Kensington Expert Mouse trackball when it first came out many years ago. All versions have four buttons. The newer versions have added a scroll ring around the trackball. As a result, I have been able to avoid RSIs. The added benefit is that users of these trackballs have enhanced functionality in day-to-day operations, from programming, to browsing, to graphics work, and gaming.
Since you mentioned physical discomfort with a dangling ring finger I must stress that you try out the Kensington Expert Mouse (can be bought new as cheap as $60). The ring finger rests on the right button or the upper-right button depending on your preference.