Actually 4.20 is a major upgrade, check this link for 4.20 Usage Tips . The input module includes expanded data arrays to handle hydrodynamic loading, as well as three new piping codes.

Because of this data expansion, 4.10 would choke on a 4.20 input file, even if you don't use these new features. So your initial conclusion is correct.

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Regards,
Richard Ay (COADE, Inc.)