You can use a script like the following for the custom calculate script for the E text field:
(function () {
// Get the values from the input fields, as strings
var s1 = getField("A").valueAsString;
var s2 = getField("B").valueAsString;
var s3 = getField("C").valueAsString;
var s4 = getField("D").valueAsString;
var s = "";
if (s1) s+= s1 + "\r";
if (s2) s+= s2 + "\r";
if (s3) s+= s3 + "\r";
if (s4) s+= s4;
})();
This separates each field value with a single carriage return (\r), so field E should be set to multiline. There's a potential for the last character to be a carriage return, and if you want to prevent this the code would have to be changed slightly.