IIF is an Inline If statement. Since you need to incorporate functions, it may be helpful if you isolate those first. Then, try and incorporate them into your IIF().
You started that process with trying to get your date sorted out.
I think the date portion would be like this: Date()-1825>=[Order Date]
When counting things like days months and years, it is best to use functions that will take into account things like leap years and months with 28 or 30 days. Maybe you can test the following VBA out in a form. Check to see if it is counting the correct number of years.
Code:
if DateDiff("yyyy",Me![Order Date],Date()) > 4 Then
msgbox "Out of warranty"
end if