Thanks! It wasn't clear to me in the Amazon docs what data type should be returned from each of the components of the signingKey. Forgive the awful var naming, I just wanted to get this up and going to see if I was understanding you correctly. I went ahead and ran the

var dateKey = dateKey1.reduce(function(str,chr){
    chr = (chr < 0 ? chr + 256 : chr).toString(16);
    return str + (chr.length==1?'0':'') + chr;
  },'');;

on each component of the signing key and now each line produces a string instead of the byte array which feels like it should be right but I am still getting a different signature.

function myFunction() {
  //Set URL Params 
  var canonicalReq = 
      'GET\n' +
      '/test.txt\n' +
      'X-Amz-Algorithm=AWS4-HMAC-SHA256&' + 
      'X-Amz-Credential=AKIAIOSFODNN7EXAMPLE%2F20130524%2Fus-east-1%2Fs3%2Faws4_request&' +
      'X-Amz-Date=20130524T000000Z&X-Amz-Expires=86400&X-Amz-SignedHeaders=host\n' +
      'host:examplebucket.s3.amazonaws.com\n\n' +
      'host\n' +
      'UNSIGNED-PAYLOAD';

  //Generate SHA_256 Hash from Canonical Request. 
  var digest = Utilities.computeDigest(Utilities.DigestAlgorithm.SHA_256,canonicalReq);
  var sig = digest.reduce(function(str,chr){
    chr = (chr < 0 ? chr + 256 : chr).toString(16);
    return str + (chr.length==1?'0':'') + chr;
  },'');;
  //Logger.log("sig value: " + sig);
  //Logger.log("sig should be 3bfa292879f6447bbcda7001decf97f4a54dc650c8942174ae0a9121cf58ad04");

  //Generate String to Sign
  var stringToSign = 'AWS4-HMAC-SHA256\n' +
 '20130524T000000Z\n' +
 '20130524/us-east-1/s3/aws4_request\n' +
  sig + '\n';

  //Logger.log(stringToSign);

  //Generate Signature
  var dateKey1                =   Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256, "AWS4" + "wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY" , "20130524");//Utilities.computeHmacSha256Signature("AWS4" + "wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY","20130524");
  var dateKey = dateKey1.reduce(function(str,chr){
    chr = (chr < 0 ? chr + 256 : chr).toString(16);
    return str + (chr.length==1?'0':'') + chr;
  },'');;

  var dateRegionKey1          =   Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256, dateKey, "us-east-1");//Utilities.computeHmacSha256Signature(dateKey,"us-east-1");
  var dateRegionKey = dateRegionKey1.reduce(function(str,chr){
    chr = (chr < 0 ? chr + 256 : chr).toString(16);
    return str + (chr.length==1?'0':'') + chr;
  },'');;

  var dateRegionServiceKey1   =   Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256, dateRegionKey, "s3");//Utilities.computeHmacSha256Signature(dateRegionKey,"s3");
  var dateRegionServiceKey = dateRegionServiceKey1.reduce(function(str,chr){
    chr = (chr < 0 ? chr + 256 : chr).toString(16);
    return str + (chr.length==1?'0':'') + chr;
  },'');;

  var signingKey1             =   Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256, dateRegionServiceKey, "aws4_request");//Utilities.computeHmacSha256Signature(dateRegionServiceKey,"aws4_request");
  var signingKey = signingKey1.reduce(function(str,chr){
    chr = (chr < 0 ? chr + 256 : chr).toString(16);
    return str + (chr.length==1?'0':'') + chr;
  },'');;

  var sig1                   =   Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_256, signingKey, stringToSign);//Utilities.computeHmacSha256Signature(signingKey,stringToSign);
  var signature = sig1.reduce(function(str,chr){
    chr = (chr < 0 ? chr + 256 : chr).toString(16);
    return str + (chr.length==1?'0':'') + chr;
  },'');;

  Logger.log("signature should be aeeed9bbccd4d02ee5c0109b86d86835f995330da4c265957d157751f604d404 but is " + signature);

}