There is no directly supported a fixed ratio layout in Qt4. If we want to make a fixed ratioed layout, we must derive a new class from QLayout and override some of its functions. These functions not only for ratio purposes, but also support for adding and removing new items to the layout.
There is a subject which expains this process at Nokia forums. You can find it here. The design in that subject for a centered square widget. I added some functions to gain a golden ratio or an A4 paper ratio. You can see an example application in the video below.