If you need a PCM audio file larger than 4GB, save as a 'Sony Wave64' <.w64>
I am not familiar with the 'Steam edition' of Sound Forge 12 or if it has the option to render <.w64> fles. SF Audio Studio 12 in general was a unique build, but the regular Magix version of SF Pro-12 has the <.w64> option.
btw, welcome to the Magix Sound Forge users community @Jonas-Viklund.