Road warrior territory. Many busy multi-taskers desire the ability to write in multiple colors, modes and even technologies. Hence, the multi-pen.

Typically, a multi-pen will have two ballpoint tips, one each in blue and black, a pencil and a pda tip. This would comprise a 'four pen'. Some brands will even throw in an additional ballpoint color and the total will be five points contained within the single pen.

Sheaffer even has a multi-pen which is a combination ballpoint and highlighter. Monteverde has a pen named M3 which on one end is a ballpoint and at the other it is either a MEGA InkBall liquid ink roller ball or a full fledged fountain pen nib. You choose. The choices will continue to expand.