If you are entering the initial data in a global field you don't need any variables.
You can use this kind of code:
Enter find Mode 
Set Field [YOurTable::DateField ; YourTable::GlobalMonday & "..." & YourTable::GlobalMonday + 4 ]
Set Error Capture [on]
What you are trying now, within a stored find request doesn't work because Filemaker can't evaluate a calculation within a stored find request. You could use:
Set Variable [$DateRange ; value: YourTable::GlobalMonday & "..." & YourTable::GlobalMonday + 4 ]
and then you have a single variable you can use in a stored find request, but I prefer the first method I posted as it enables me to see all the criteria used in the find without the added inconvenience of double clicking a script step to open up an extra dialog to see what was specified.
For more scripted find examples using the first method, see: Scripted Find Examples