Kindle 3 has the ability to do simple calculations with certain keyboard characters in its search box. Doubtless, K4 also has something similar. but I doubt that is really what you are after. In which case, you could try out the free online bog standard:
http://www.kinstant.com/calculator/
Incidentally, I noted accidentally (while typing this) that you can use two fingers on this calculator by using the 5-Way to move the cursor with a right-hand finger and the spacebar with a left-hand finger to actually enter the characters. This avoids the fiddle of making sure you hit the central key on the 5-Way.
In the similar threads box below this thread, you will see further calculator links. Also:
http://www.amazon.com/dp/B004XN70ZI/
Now the thing is about the above 1 dollar app, that it is probably worth such a small outlay - if it weren't for the exasperating fact that it isn't available (yet) to most countries in the World.