String max size for 722MB xml file

I have a ByteArrayOutputStream which holds a byte representation of an XML with 750MB size.

I need to convert it to String.

I wrote:

ByteArrayOutputStream xmlArchive = ...
String xmlAsString = xmlArchive.toString(UTF8);

However although I am using 4GB of heap size I get java.lang.OutOfMemoryError: Java heap space

What is wrong? How can I know which heap size to use? I am using JDK64 bit

UPDATE

I need it as String in order to remove all the characters before "

Currently my code is:

String xmlAsString = xmlArchive.toString(UTF8);
int xmlBegin = xmlAsString.indexOf("<?xml");
if (xmlBegin >0){
        return xmlAsString.substring(xmlBegin);
}
return xmlAsString;

I then convert it again to byte array.