Dear dPritchard,
As NTAG 424 DNA is NFC Forum Type4Tag, you need to select NDEF Application first and then perform ISOReadData command. On top Read Access right (e.g. 0x0E) to NDEF file needs to be granted.
Try after your GetVersion command, like this (interface APIs):
byte[] NTAG424DNA_NDEF_APP_NAME =
{(byte) 0xD2, (byte) 0x76, 0x00, 0x00, (byte) 0x85, 0x01, 0x01};
intag424DNA.isoSelectApplicationByDFName(NTAG424DNA_NDEF_APP_NAME);
retData = intag424DNA.readData(02, 0x00, 0x00);