to short will mean that the DateTimePicker will use the sort format of the current culture on the machine (if not otherwise set). So on Mac the default culture is with only two digits and on Windows is with 4 and that is why the difference can be observed in the DateTimePicker.
Nevertheless if you want to set the year to always 4 digits you can do so by setting the Culture property of the DateTimePicker:
CultureInfo cultureInfo = new CultureInfo("en-us");
DateTimeFormatInfo dateInfo = new DateTimeFormatInfo();
dateInfo.ShortDatePattern = "MM/dd/yyyy";
cultureInfo.DateTimeFormat = dateInfo;
dateTimePicker.Culture = cultureInfo;
Hope this would help.
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