Each half hour, the candle burns away 3/4 inch of its length. Therefore, after 1 hour, the candle would have burned away 1.5 inches (2 half-hour periods * 3/4 inch burned in each period), leaving it with a length of 0.5 inches.
After 2 hours, the candle would have burned away another 1.5 inches (4 half-hour periods * 3/4 inch burned in each period), leaving it with a length of 0 inches. At this point, the candle has completely burned down.
Therefore, after 4 hours, the length of the candle would be 0 inches.