You have the 4¢ Martha Washington from the years 1923-30. It is a common stamp and is worth no more than a couple of dollars whether used or unused. Taking the easily identifiable varieties first;

The stamp has the letters Kans. on it

The value of this stamp (#662) can be found by clicking here.

The stamp has the letters Nebr. on it

The value of this stamp (#673) can be found by clicking here.

The stamp has no perforations on top AND bottom

The value of this stamp (#601) can be found by clicking here.

The stamp has perforations on three or more sides
and you count ten perforations along the bottom

The value of this stamp (#585) can be found by clicking here.

The stamp has perforations on three or more sides
and you count eleven perforations along the bottom

To determine the difference in the perforations between #556 and #636 you will need a copy of the very common 1932 3¢ Washington (#720). A picture of which is shown below. Follow the instructions below

The value of #556 can be found by clicking here.
The value of #636 can be found by clicking here.