4-to-10 Decoder truth table
If there is four input variables it is possible to make 16 different combination. That means 4:16 decoder is also possible. But that doesn't mean when ever at input side there is four variables there should be 16 outputs.

Decoders are designed based on the application requirement. If number of output possibilities is in between 9 to 16 we have to go for 4 input variables. 

For example if we want to make a BCD decoder, there is only 10 possible output combination. In that case we will use four variable at the input side. Here input combinations 1010, 1011, 1100, 1101, 1110, 1111 is unused.