I just had a look at this page: WinTV for Windows 7
I'm under the impression that it isn't the software that's limited to less than 4GB, but the hardware (the PCI card). If I'm right (I suspect the idea is to prevent the card from mapping its I/O space above the first 4GB), then you'll have to rely on the BIOS to limit the available RAM before even booting into Windows.
Whether it's the hardware or its associated software that has this limitation, I would think an upgrade is in order as this limitation is simply a sign of dated hardware/software and doesn't make much sense.
I can't think of any other solution except maybe running the software in compatibility mode for XP or earlier. Doubt that'll do it, though.