This can be done using the Crop function. Activate the function using the corresponding tab. The function is visible by the colour of the tab and a frame for cropping.

Use the mouse to position the frame. Move the cursor into the frame. When it changes to a cross sign, move the frame by dragging and dropping.

Then you can set the frame's width or height by dragging the edges. Do this by positioning the cursor over the edge you want to adjust until it changes into a double arrow. Then drag the edge to the position you want it in.

The second dimension is to set the required aspect ratio. Do this by adjusting the figure (to the left of the editing screen).

The aspect ratio should be 3:4. This results in a height = (width:3) x 4 or width = (height:4) x 3.

Optimise the image selection by repeating these steps.

Save the changes with 'Apply'.