The current implementation of NTFS only supports about 4 billion clusters on a volume - so at 512 bytes per cluster the file system would be limited to 2TB in size. That's an NTFS limitation and has nothing to do with whether you use GPT or MBR (except, of course, that with MBR you'll never have enough space available to create an NTFS volume larger than about 2TB anyway).
4K is a perfectly fine cluster size for NTFS - there's really no reason to fret about it.
Good to know. I only have the one 3TB drive works great but on occasion gives me alot of trouble. I put it back in its external houseing i hope it doesn't get write errors again like it was doing. Damn seagate adapters suk, there drives are great but there adapters are horrible. I had the drive in my PC for a while and it ran great better letency then my 1TB drives and faster.