You can present a simple UI that allows all four values to be set at once. I created a mockup that illustrates this principle. Note that I haven't particularly focused on making sure that the UI doesn't always present exactly 100% (due to rounding). I leave that as an exercise to the reader.
This control scheme is an "inverted control scheme," where the closer you move the selection point to a setting, the stronger the effect. While this is usually intuitive (the picture hardly does it justice), you might also opt for a more straight-forward implementation:
I've also created a mockup for this as well. You'll notice the big difference is that now, instead of "moving towards" a control to increase the value, you instead "increase the area size." Both UIs should be equally intuitive for users, so I would consider this more of a personal preference.
It was noticed that some combinations of values can't be selected. This wasn't my intent, but more to point out the fact that if all four values are interlocked, that the user should have a visual association to the four controls and their values. This means that sliders cause cognitive dissonance.
I've created a third (and final) version of a UI that would allow users to select any combination of values where the total will always equal 100%. This version still outlines the importance of linking the four controls together visually so that users can determine how moving one point will affect the others.