Now there is nothing wrong with having four corners the same in a painting. It makes the painting look more like a vignette, a technique used in the past in which the border of the image is blurry. But, if you want a more dynamic looking painting, it's a good idea to pay attention to all four corners and make sure they are all different. I does not always work perfectly but it adds "un je ne sais quoi" to your painting. Try it next time. It also reminds you to use gradation and not paint a large area all the same tone or colour. It's good practice!