If you inspect the HTML behind the "columns format" of your prompts you will see each columns of prompts his a <td> of a main table. So you can add some custom CSS to change alignment and size of that main table and the 4 <td>.
You can probably also try a less "clean" solution like adding lot of spaces in front of the longer label of the 2nd, 3rd and 4th columns to "push" the others columns a bit more.
There aren't lot of available settings for how prompts must look like, but it's HTML, so with JS or CSS you can change the way it looks like...
I would only advice to not make anything too complicated also because you have the risk the HTML code change with a future release of OBIEE.
Also try to just align the prompts centrally, they will maybe look a bit better with less empty space on the left.
I managed to make checkboxes go horizontal instead of vertical using JS/CSS a while back but unfortunately every time I changed the value in a prompt it would refresh them and not redo my JS. I stopped there because I was doing it more for interest rather than anything else.
Daniel Willis : time to move to 220.127.116.11, horizontal checkboxes / radio buttons are there out of the box (Rittman Mead Consulting » OBIEE 18.104.22.168: New Features for Front-End Developers and Business Users > "Horizontal Layout for Radio Button and Check Box Dashboard Prompts")
To spread out between columns in PROMPTS, is possible to add a character that is not visible but takes up space (normal space is ignored), use alt + 255.
While holding down the alt key, write 255 in the numeric keypad (on the right). Release the alt between each character.