I noticed that you are not using now R1C1 reference style

There are 2 options, You can use this formula:
=IF(LEN(Sheet1!A4)>0
;Sheet1!A4
;"")
you might need to change the "
;" separator to "
," separator, depending on your regional settings.
Or, in excel options-complex-show options for this worksheet : uncheck the option -show a zero in cells with zero value