Creates surface defined by four splines. The order of picking the splines matters - they should form a closed loop. If you get a weird looking mesh, try once again starting with another of the four splines:
Note that if any of the splines are animated, the resulting mesh is animated, too.
Copy the .ms file to your scripts\startup folder or run as a script if you only want to try it out. The object primitive will be available under Examples in the Create panel.
There's also an additional .mcr file - after you've installed the .ms file, it's enough to run the .mcr file once. It creates a Make Surface From Selection item in the Shortcuts category under Customize UI, which you can then add to a quadmenu or assign a shortcut to it (that's what I'm showing in the preview gif). To use it, select the four individual splines in a circular manner (counter-clockwise is preffered but will work with either - you can always flip normals) and press the button/keyboard shortcut.