4-prong outlet exposed?
If this is 2 duplex recptacles installed side-by-side in the same box, you might be able to use a larger cover plate.
Cover plates come in 3 sizes, standard, intermediate, and jumbo. According to the Leviton (a manufacturer) book, a standard 2 gang (double duplex) plate measures 4.5" high, and 4.56" wide. The intermediate is 4.875" high, and 4.94" wide. The jumbo (it certainly earns its name lol) is 5.25" high, and 5.31" wide.
The intermediate plates look ok, if it will cover the gap, use that one. The jumbos look huge, sort of out-of-place, but it might be the best (least expensive) solution.