Can't Partition 4TB WD Red drive under Win7 64

I bought a new Western Digital Red drive for an older computer, a Dell Studio 540s, a Q8200, from about 4 years ago. Previously, I had a 2 TB drive that had worked ok.

My problem is that I can't access all 4 TB. When I install new Win7 64 Home premium, Windows sets aside 100MB for System, 2 TB for the rest, and the remainder appears as unpartitioned space. When I try to use disk management to create a new simple partition, that command is greyed out. I cannot figure out any way to get at that additional space. I would be happy to either have a 4TB partition, or two 2 TB partitions. Anything so that the space is not wasted.

I read somewhere that I should reformat it as a GPT drive, whatever that is, but where the instructions indicated to do that in disk management, I could not. I believe the option was greyed out there also, or something else didn't let me.

Note, since installing Windows, I have not put any other programs on here, so if I need to do a full reinstall, I could do that too (I've already done it twice).

Any thoughts?

Thanks. Sysinfo below.

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Home Premium, Service Pack 1, 64 bit
Processor: Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz, Intel64 Family 6 Model 42 Stepping 7
Processor Count: 8
RAM: 16366 Mb
Graphics Card: AMD RADEON HD 6450, 1024 Mb
Hard Drives: C: Total - 1413087 MB, Free - 449833 MB; K: Total - 1907695 MB, Free - 1427775 MB;
Motherboard: Dell Inc., 0Y2MRG
Antivirus: Norton 360, Updated and Enabled