As you can see on the object to the left in the picture below the horizontal lines is perfectly centered relative to each other on the horizontal axis. How can I do the same on the object to the right? I have selected the 2 nodes on the top of the object but how do I center them vertically relative to the bottom 2 nodes?
I guess your actual image is more complex. Otherwise you would draw the wanted shape right from the start. But to answer the question I would turn snap to nodes ON and draw a straight line between the top nodes with the Bezier tool. Then I would align the new line and the shape. Finally I would move the top nodes to the ends of the line with the node tool. See the cartoon below:
The blue line and the original shape were aligned by applying Object > Align and Distribute > Center on vertical axis.
If the actual shape is something more complex or the top side reaches further to the left or further to the right than the bottom side, you can draw another line to the bottom and align the lines at first. Move then the original shape and its top corners.