Robin that isn't true.
If you have Snow Leopard or Lion then any native 64bit apps will run in 64bit with 4GB of ram. It worked perfectly well on my MBP before i upgraded to 8GB and you can check what your apps are running in the Activity Monitor
You can also force it to run in 64bit constantly on Snow Leopard by holding 6 & 4 when you start up.
....OK, then I'll elaborate:
If you run a 64-bit App with only 4 GB RAM installed on your Mac, it'll only use 4 GB of the installed RAM regardless. Therefore you should have 8 GB RAM to run 64-bit Apps more efficiently. This certainly applies to Apple's Logic Pro 9.1.5.
Furthermore the OS can use a Bridge for 32-bit plug-ins within a 64-bit App but it can compromise performance.
Sorry if I wasn't clear.
And you can also run an App individually in 64-bit if you Get Info on the App icon and deselect the "32-bit" box if the App's Get Info window has the option.