The safest, but slowest, way to defrost a roast is to thaw it in the refrigerator. A 4-pound roast takes about 24 hours to thaw in the refrigerator. Juices seep out during defrosting, which the meat generally re-absorbs.
Never thaw meat at room temperature because freezing the meat does not kill bacteria, which grow rapidly to dangerous levels at room temperature. You may defrost a roast in cold water if it is sealed in a leakproof bag. Replace the water every 30 minutes for two to three hours. This method typically does not allow the meat time to reabsorb the juices, resulting in a drier roast. Cook the roast as soon as possible to keep it safe to eat.
If you defrost the roast in a microwave, cook it immediately. Microwave defrosting cooks the outside of the roast and dries it out.