Unit rate is the quantity per one. In this case it is how many candles per 1 box.
You divide everything by 4 to get 12.5 candles per 1 box (or 12 candles in 4 boxes and 2 left over.)
The ratio between candles in boxes would just be 50 candles to 4 boxes, or 50:4. You can simplify it to the unit rate of 12.5 candles to 1 boxes, or 12.5:1