Few basic questions on combinational logic
Does that mean the first decoder has 4 inputs and 16 outputs ? 


No, it means the whole circuit you are designing has 4 inputs and 16 outputs.


  If so why the answer to this question has for the first decoder a 2 inputs and only 4 outputs


Because your instructions are to build your circuit "using 2-to-4 decoder[s]". You need to use 2-to-4 decoders as the components in the circuit you are designing.