This is really easy if you understand video and delivery systems and you communicate with the AV team. Three simple steps.
- Contact the AV team and/or Display system manufacturer and determine the exact specifications for the file they need you to deliver because if you try and guess there's a better than even chance that you'll look stupid
- If your display size is really 4608 X 1080 then set up your comp to those dimensions rather than trying to make your own custom pixel aspect ratio because trying to compose or judge anything when working in non square pixels, even with PAR correction turned on, is miserable
- Nest your odd sized comp inside a standard HD comp and size horizontally (Alt/Option + Shift + Ctrl/Cmnd + h) and then render the standard comp to the specifications you have from your AV and/or display system manufacturer
If you don't know what needs to be delivered run a quick test by creating a 4608 X 1080 comp that is 5 seconds long, add a perfect circle and a perfect circle square to the comp by creating a square solid 900 X 900 pixels, duplicating the solid layer, position one of the layers on the left side, the other on the right, double click the Ellipse tool with the shape on the right selected to create a circle mask, then animate the rotation of both layers so you have 1 + 90º rotations starting at the first second and ending at the fourth so you can see the shape at rest, a full rotation + 90º then the layers at rest, drag the comp into a standard HD comp, scale it to fit horizontally (layer menu or keyboard shortcut), render and send it off to be tested. The circle should not wobble and the square should not turn into a rectangle when it has been rotated 90º.
That's it. Media players for the most part completely ignore pixel aspect ratio metadata and I don't know of any that will have any idea what to do with a custom PAR in the metadata. That's all PAR ever is - metadata. It does not change the pixels.
Hope this helps.