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Serial2.write("\x04"); 

That will write one character to Serial2, but you do not need to specify a length. Since it is only a single character, it is better to use single quotes '\x04' because the computer will store "\x04" as a string, using two bytes of storage for the \x04 followed by a null.

As long as the text does not contain \x00 you do not need to specify a length, that is only needed when there is \x00 in the text because that value is used to indicate the end of a text string.

I should have paid more attention to my programming class in college.... In 1982. It's working and I will keep tinkering and learning. Thank you for your patience.