IIF should be fine.
Syntax I'm familiar with has only ever been : IIF(resn70='4',0,lqty70*litr35/iscf35/100)
That is, comma separators between arguments.
Also in case relevant, the '4' is only suitable for a text field. I'm wondering if resn70 is a numeric field?
If so, try IIF(resn70=4,0,lqty70*litr35/iscf35/100)