In order to establish adverse possession of the portion of the vacant lot,  you have to establish continuous, hostile, open, notorious and actual possession for the statutory period.By maintaining the portion of the vacant lot, you have established that  your use of the property has been continuous.  It has been hostile (in derogation of the true owner's (developer's) rights).  It has been open and notorious which means the owner (developer) has had notice of your use of the property which you have openly maintained.The problem is that in Alabama, the statutory period for adverse possession is ten years.  Therefore, since only four years have elapsed, adverse possession for the statutory period has not been established.  Since you have satisfied all the other elements of adverse possession, if you continue for another six years, you will then have established adverse possession of the portion of the vacant lot.