m4=b4 only if

[Derek]

I want to copy data from on cell to another cell, but only if a third cell
has data in it.

 

[Niek Otten]

And what should the value of "another cell" be if the third cell has no
data?

 

[Joe User]

I want to copy data from on cell to another cell,
but only if a third cell has data in it.

A little light on details, to say the least. "You get as good as you give".
Perhaps something like (in M4):

=if(A4="", "", B4)