Problem with calculated column in Lists

Hi,

I'm having a bit of a struggle getting a calculated column doing what I want.

So here's the issue;

I have 6 columns

Column1, column2 and column3 have timestamps in them

Column4 and column5 can have a value or be empty

Column6 is a calculated one

For now column6 has following formula to show the minutes between the 2 timestaps

=([Column2]-Column1)*1440

This is working fine.

But now I'm looking for a solution that, in case there's a value in column4, column6 just needs to show (copy over) the value that's in column4 and not do the calculation anymore.

Thanks in advance