But I want it! Shaped Swimsuit from Land's End

Now I might be getting a little a head of the seasons here (I'm still freezing in my sweaters and scarves) but this "structured" swimsuit really appeals to me! I like that it's a halter, that it has underwire (and the option of adding padding to your chest ;D), and that it has this classic look.

I don't swim much, so what happens is that I buy a swimsuit and it lasts me years. And by years I don't mean a handful, I mean years and years.  So I rationalize that spending $29 on a flattering swimsuit will pay itself back in a few years (haha).

However I don't understand the sizing. Is "4" seriously the smallest size they have? O.o; And what is that equivalent to?  And what do they mean by short torso? How do I know if I have a short torso? What is a regular torso length?  What about people with long torsos? o.O; 

Any ideas?