One way to interface basically anything to a computer by "keypresses" is to hack an old USB keyboard.
In a USB keyboard there's a tiny little printed circuit board with two groups of connectors that are normally connected to all the keys on the keyboard. These connectors are organised in a matrix of X rows and Y coloumns. By shorting one connectore from the rows group to one connector in the coloumns group you simulate a key press. It's just a matter of trial and error to find out which connectors to short together to produce any kepress you like.
Unfortunately different USB keyboards have different layout of the connectors, so it is not possible to give any specific advice on which connectors to use.
When you know which connectors to short to produce the desired keypresses, you can hook a 4066 Quad analog switch IC up to 4 digital pins on the arduino board and the 4 switces on this IC can be used to short 4 connector pairs in the USB keyboard PCB, and in that way simulate 4 different keypresses. By closing more of the 4066 switches at the same time you can even simulate things like "SHIFT + F5" or whatever key combination you like.
This method can also be used to build alternative input devices that can replace an ordinary keyboard. You can for instance build a box with 4 large switches that will allow people to press only 4 of the keys on a keyboard connected to a computer used to control some part of an installation or whatever.
MikMo
EDIT: Be aware that by using the above method you can produce key press combinations that are not normally possible or allowed, this might crash some computer programs i think. In the simple tests i have made until now this has not happend.