Your frozen roast is like something out of a science fiction story. The bacteria on it doesn't die in the freezer, it's in suspended animation, and when the temperature starts rising, the bacteria starts spreading. To keep a 4-pound roast safe during thawing, minimize time in the danger zone -- between 40 and 140 degrees Fahrenheit. There are a few safe ways to do so -- defrosting at room temperature isn't one.
The Best Defrost Option
Put your frozen roast in the refrigerator for one to three days -- five days at the most. A 4-pound cut takes about 24 hours to thaw, but give it extra time if possible, just to be sure. This is the slowest method, but the safest and best for maintaining quality. Juices seep out of meat during thawing, but this method provides enough time for much of them to be reabsorbed.
The Second-Best Defrost Option
If your roast is in leak-proof packaging, submerge a frozen 4-pouind cut in cold water for 2 to 3 hours. To hold bacterial reproduction at bay, replace the water every half hour with colder water. This is faster than refrigerator thawing, but requires more attention and results in more moisture loss. You may flirt with too much time at a risky holding temperature, so cook the food promptly after thawing.
If You Must
It's safe to thaw meat using your microwave's defrost function, but it's not a great option. A 4-pound roast takes many minutes, and the exterior starts cooking. Your final product is likely to be overcooked near the outside. Because it's partially cooked while defrosting, finish cooking the roast immediately afterwards.