The picture is not so good. The surgical area have some edema ( natural in this fase) , equimoses in the skin. Is difficult to define if this is an hematoma or seroma. Is better to return to your doctor and cheek this.
Its very difficult to say without an exam. Its not uncommon to get bruising around the incision site following an augmentation which is most likely what you are seeing. However if your breasts feel different and one side is more swollen,tender and harder you could be dealing with a hematoma. I suggest you contact your surgeon.
Do you think it's a Hematoma? It's not hard and a little painful. Surgery 4 days ago.
Always best, to have your plastic surgeon evaluate your situation. Most likely, having said that, you are dealing with a bruise. This should resolve with time. Again, your surgeon will be your best resource. Best wishes.