Under eye dent when smiling since I had filler in this area 4 years ago, how do I fix it? (Photos)

I wish you also included a photo of your eyes looking down and took a photo from below to show the under eye area when you are looking down with your eyes.It definitely looks like a filler. It would don't hurt at all to try to dissolve it a bit. You may also try little botox to the bunny lines to lessen the "bump" formation.
This will require a little bit of hand holding and not everyone is up for it. If it really bothers you, then you need to be prepared to have a few dissolving sessions and maybe will need a little bit of more filler later.
Good luck.

At least from the photos and the history of filler use in the area, the problem is likely one that has been nicknamed under-eye "puffy pillows," or more specifically infraorbital lymphedema (lymphatic obstruction) likely secondarily to the use of an inappropriate choice of filler for the area and/or too superficial injection of the filler in that area. Unfortunately, it tends to be a difficult problem to treat and a persistent one, however, I have successfully treated a number of individuals who have come to me with this problem using an algorithm of varying combinations, concentrations and amounts of intralesional anti-inflammatory Agents. You would be wise to consult with an experienced, board certified cosmetic dermatologist. Hope this helps and best of luck.

Provided the filler was a hyaluronic acid filler, hyaluronic acid filler will remove this. I would not just remove this, I recommend removing as much of the retained filler as you have in your cheeks. Aging filler is not helpful. It is not clear to me if the filler ever actually disappears on its own. I have seen patients with issues from services that were performed 15 years before. When you inject the appropriate amount of hyaluronidase, these issues disappear instantly. It is essential for you to see a physician or surgeon who routinely works with hyaluronidase to remove retailed filler. I tell my patients prior to starting hyaluronic acid fillers that they have a life cycle. The filler needs to be topped off at the end of 9 to 12 months but after 3 years of service, the oldest filler will begin to retain water and result in distinctly unnatural volume effects that are not aesthetically helpful, For this reason, I recommend removing all of the filler when this occurs and starting the cycle over again. You always want the least amount of filler in your face that accomplishes your goals.