4 inputs and 1 single output

Mmmm ... Your JavaScript code is fine, but I would recommend JQuery, which is a more powerful library with more features. You could divide the input's into groups, and then iterate each one, your code with JQuery would be:

$('#prueba').click(function() {
    var suma = 0; var multiplicacion = 0;

    $('#suma input').each(function() {
        suma += $(this).val();
    });
    $('#mult').each(function() {
        multiplicacion *= $(this).val();
    });
    $('#resultado').append('Suma es: ' + suma + '<br/>');
    $('#resultado').append('Multiplicación: ' + multiplicacion);
});

I can think of an easier way to do what you want, I do not know what your intentions are, but it makes it easier for me by placing only two input and then you put buttons to choose the operation, that is, a button for addition and another for multiplication , you would save many input, to each button you assign a function that carries a variable to be worth one or two in this way you know which button was clicked, with a simple if you verify it you perform the operation and with a sentence you enter the value returned to the input result and you would save much code, that code you posted is very entangled for what you want to do that is quite easy, I'm on the phone but I will try to put some code JS.

//declara las variables

var valor1 = document.getElementById('input1').value;  
var valor2 = document.getElementById('input2').value;  
var operacion = 0;
var resultado = 0;

//si vale 1 es suma si vale 2 es multiplicacion

if (operacion==1){
    resultado = valor1 + valor2;

    //con esta sentencia insertas el resultado en el input
    document.getElementById('inputRespuesta').value=    resultado;  
}

Luck

You could simplify putting a button for addition and another to multiply by means of functions with only two inputs

HTML

<input id="primerValor" type="number">
<input id="segundoValor" type="number">

<input id="resultado" type="number">

<button onclick="sumar()">Sumar</button>
<button onclick="multiplicar()">Multiplicar</button>

for practical purposes you could add the function directly with onclick

JAVASCRIPT

var resultado = document.getElementByID("resultado");
function sumar() {
    var primerValor = document.getElementByID("primerValor");
    var segundoValor = document.getElementByID("segundoValor");
    resultado.value = primerValor.value + segundoValor.value;
}
function multiplicar() {
    var primerValor = document.getElementByID("primerValor");
    var segundoValor = document.getElementByID("segundoValor");
    resultado.value = primerValor.value * segundoValor.value;
}

Another way (and with best practices) would be to put the id buttons and add an event listener for each one HTML

<input id="primerValor" type="number">
<input id="segundoValor" type="number">

<input id="resultado" type="number">

<button id="sumar">Sumar</button>
<button id="multiplicar">Multiplicar</button>

JAVASCRIPT

var resultado = document.getElementByID("resultado");
var sumar = document.getElementByID("sumar");
var multiplicar = document.getElementByID("multiplicar");

sumar.addEventListener("click",function () {
    var primerValor = document.getElementByID("primerValor");
    var segundoValor = document.getElementByID("segundoValor");
    resultado.value = primerValor.value + segundoValor.value;
},false);

multiplicar.addEventListener("click",function () {
    var primerValor = document.getElementByID("primerValor");
    var segundoValor = document.getElementByID("segundoValor");
    resultado.value = primerValor.value * segundoValor.value;
},false)

I also think that the way you have tried is not the most efficient, the options that have given you the most successful seem to me those of Emiliano, but there are a couple of errors in your code that would prevent you from work correctly, the first is that when selecting the elements in js by the id the d does not have to be in uppercase, and the second is that the sum is not done correctly because js interprets the elements as strings and not as integer , you would have to convert them with parseInt (), I would also recommend doing it in multiplication to avoid possible errors, your code should be like this:

var resultado = document.getElementById("resultado");
function sumar() {
    var primerValor = document.getElementById("primerValor");
    var segundoValor = document.getElementById("segundoValor");
    resultado.value = parseInt(primerValor.value) + parseInt(segundoValor.value);
}
function multiplicar() {
    var primerValor = document.getElementById("primerValor");
    var segundoValor = document.getElementById("segundoValor");
    resultado.value = parseInt(primerValor.value) * parseInt(segundoValor.value);
}

<input id="primerValor" type="number">
<input id="segundoValor" type="number">

<input id="resultado" type="number">

<button onclick="sumar()">Sumar</button>
<button onclick="multiplicar()">Multiplicar</button>

Anyway there are more methods of getting what you want, to me this seems right but if you want you can try other ways.