According to the attribute definition for a4j:commandButton
the default value for the attribute type is "submit". I've found that to be incorrect. The default appears to be "button".
The differences affects what action happens when a user presses the entry key after completing a form. I found that a h:commandButton further down my page was being triggered because a4j:commandButton was rendered as <input type="button"... I changed a4j:commandButton to be type="submit" and this appears to cause the correct button to default.