You could applique them onto a background fabric, then cut the background fabric into equal sizes to create a pieced quilt. Or you could applique them onto a large piece of background fabric (the size of a quilt, or the inner portion of a quilt).
You could also add a border fabric around each one and mount them on frames (either quilted or unquilted), making 4 different pictures to hang.
Whatever you do, you will need a seam allowance. Perhaps you could carefully unsew some of the hand applique near the edge, turn under the edge of the applique more, and re-stitch that part of the applique to allow for a seam allowance.
If there is a bit of fabric at the edge, say 1/8" inch, you could possibly fuse fabric onto that edge, zigzag to secure the edge of the applique to the additional fused fabric, then turn so that the fused fabric adds enough to make a secure seam allowance.