As per what i have understood by your last comment.
You should have different ids for third and fourth box i.e. #val3 , #val4 respectively.
So it would be somewhat like this.
See this updated FIDDLE of yours.
Entering the value 4,5,10,5 respectively in the 4 boxes, your subtotal would be 70 as expected.
Just a note: call the function calcSum only when all the four fields are provided. Put the conditions over there otherwise it would return NaN(not a number) even if one field is missing.
if(val1 && val2 && val3 && val4){
    // do your calculation and return the sum
}
else{
    alert("please fill all the fields :)");
}
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solved Calculations on webpage using javascript