You could use Loft and PlanarSrf.
Select the bottom two curves and PlanarSrf.
Select the top two curves and PlanarSrf.
Select the outer curves and Loft.
Select the inner curves and Loft.
There are other ways to do it, but this would work. I’m sure some Rhino gurus here could do it in one step.
Hmm if you Join up the end surfaces that works, but something isn’t quite right, it should finish the Join operation automatically after picking the ends. Can’t see anything wrong with the curves.
But for me it doesn’t as I said automatically end the Join after picking them, though it does result in a closed polysurface. And Cap doesn’t work. So one would guess something is microscopically not-quite-flat somewhere, or off in a curve.