Yea, I get that with the 6G, but, unless I'm missing something, if you do it that way you can't then dual boot, so can't use OF for syncing, and hence have to deal with disk corruption issues. For that reason I stick with 7G or 5G.
Are you saying though that this 1.7Tb limit kicks in even if you reformat with windows? Also, does device manager/disk manager also fail to see the full 4Tb, or is it just that explorer can't see it? I've had devices where explorer refused to see the full capacity but it was there in device manager.
A final thought is that Rockbox has some ability to deal with multiple partitions, but I have a feeling that that capability is either incomplete or has suffered some kind of code-rot, over the years. That might eventually offer a way past a 2Tb barrier.