I know this is an old and possibly forgotten question, but I'm going to try to answer it anyway (also I have no clue about Hinduism, so I'm going to try my best of what I can find on the web).
So from what I learned, these four objects look very similar on the outside, but if you take a look on the inside (blueprints), you can see that they vary slightly.
The Rukma has four different platforms all inside a cone shaped object. It has 2 passenger platforms (top), and two control platforms (bottom). It also has two fan blades at the front, and a light bulb (I think).
The Tripura is more of a submarine like craft, with a propeller at the front, and a ruddar at the back. Measuring 100 feet in length, it has it's entrance (I don't know what it's called, but it's like two small pillars with a hatch) in the top middle.
The Shakuna is more of a warship looking craft, but it has 3 extra fins, two at the sides, and 1 at the back. At the top is an antenna looking thing, making up half the height of the ship.
And finally the Sundara. It's similar to the Rukma, but it only has one fan at the top, and it has a wider base.
Like I said before, I don't know anything about this, so I only provided the physical descriptions. Sorry for the lack of information. Hope this helps!
EDIT: Like @MalayTheDynamo said, try asking this somewhere else where more people would have the answer. Right now, I find this site more about Western topics, but hopefully it will change in the future.