Flickering distortion or "hot spots" ?

http://stevekulpa.net/pinball/bally_disp6r.htm

Digit out or shorted on you can probably fix board. Burned or flickering digit probably a pad panel.

Junk. Get rid of it.

or keep it for a spare player-4 slot

Depends on how much it bugs you. Though, you can find good used displays cheaper than a new original.

Replacement LED displays are also available. These digits are very close to the original plasma displays.
https://pinitech.com/products/cat_displays.php

If you can live with it, move it to player 4

Those dots / spots like that are burns in the segment of the digit. There's no repairing it. The plasma glass is shot. If the digits did work, they'd look noticeably different than the other displays since you'd see dark spots when lit, odds are they'd not be as bright or they'd be arcing.

If you're ever looking through piles of displays at a pinball show flea market, it's easy to identify ones that are DEFINITELY shot.. even without the display lighting up because there's usually spots or very dark segments on just some of the digits. All the segments on all the digits should look similar, without spots, without dark burns in some of the segments. The displays may still work, but they won't look good.