Portrait Image change to Landscape image with black edges

You can do this in the Lightroom Print panel. The trick is to Print to file, not to a printer. Here's a screen grab with the important settings circled, except I forgot to circle the first option under the Print job panel where it says Print to: JPEG file. I set the file dimensions to 8" x 6" because LR won't let you go below 4". If the file size is too big, just reduce the resolution. The proportions will be correct.

IMHO this is way easier and faster than using Photoshop.

(this is an image from the Adobe MAX Bash party this past week in San Diego)

I would be doing this in Photoshop.

Open a new 'document' that is 4:3  (eg. 1920x1440pixels) Black background. (Save a Ps 'preset' or a Black 4x3 photo in Lr)

Open the 'Portrait' image in Ps (from Lightroom-  Ctrl+E) and drag "Move tool"  it onto the Black 4:3 document.

Use Transform [Ctrl+T] to resize and fit on the 4:3 document.  (Hint: To see all the Transform frame press [Ctrl+zero]. )

Save as a jpg for the digital screen.

If the Black 4x3 background was already in Lr, you could just select both Background and image and [Open as Layers in Photoshop]