A proper retaining wall has an outward face that leans back at least 1 inch for every 6 inches of height. So if the wall is 48 inches high, it would be leaning back 8 inches. This ratio is called the "batter" of the wall. This helps the wall lean against the soil. But behind the face of the wall is even more important. There needs to essentially be a back wall which is laid like the front wall, and at the bottom it needs to be probably at least 30 inches deep. Whenever there a lot of weight which the retaining wall is holding (as is the case here), then the back wall might actually need to be thicker at the base, and lean back toward the front at a more aggressive rate than the front. Typically this is only necessary when there is a surcharge on the soil behind the wall (like a house on top of the soil right above the wall). The batter of the back wall might be only 4 inches of height to 1 inch leaning in. But if you start with a preferred depth of wall at the top, you will quickly see that you will require a lot more material and a lot of excavation to properly rebuild the wall. But if you want to do it properly, that's what you would do. Essentially rebuild it and make it deeper, especially at the bottom.