If I reduce the 5.4TB drive in Windows to 5TB can I reclaim the unused space and allocate it to the 50GB drive.
No, operating system disk allocation has no impact on virtual disks.
Your configuration is referred to as a sliced array. Situations like yours are one of the reasons sliced arrays are discouraged. They also have a higher rate of rebuild failures that are unrecoverable.
There are features and methods for expanding virtual disks if there is unused space on each drive in the virtual disk. Those options are not available with sliced arrays. If you want to make the 50GB virtual disk larger you will need to delete and recreate the virtual disks. I suggest not putting more than one virtual disk on a drive(slicing) if you can avoid it.