I think the key to your solution is introducing a carriage return.  The cleanest way to do so, IMHO, is using ChrW(13).  (13 being the ASCII code for carriage return, and ChrW() being a function that returns the character for a given ASCII code)

So your formula might look like:

Local StringVar AddOut;

AddOut = {MyReport.Add1} & ChrW(13);
AddOut = AddOut & {MyReport.Add2} & ChrW(13);
IF IsNull({MyReport.Add3} = False THEN
     AddOut = AddOut & {MyReport.Add3} & ChrW(13);
AddOut = AddOut & {MyReport.Add4};

AddOut