I recommend the safest solution. But if you must use NAV 4.0 SP3, it may work on Windows 7 if you have the right build of the files.
You can find on partnersource a procedure that allow you to use NAV 4.00 SP3 on Vista or Windows 7. You must install the following patches:
-KB931841
-KB933727
-KB936602
This procedure says that if you run the NAV with SQL Option and SQL 2005 Server, you have to set trace flag on the SQL service. You need to do this:
-Start SQL Server Configuration Manager.
-Click the Properties tab.
-Click Advanced.
-Click Startup Parameters, and then add the following trace flag: T4616
-Restart the Microsoft SQL Server service.
If you still have a problem, you can to download virtual machine + XP mode from Microsoft site and install it for NAV client.