If the drive is showing UNALLOCATED, either the drive crashed (so the MBT is broken and 'no data' exists on it), you used a different OS (this 4TB was from a MAC and now you plugged it into a Windows PC) for it initially, or 'this' PC (as opposed to where the 4TB got filled up on) doesn't know how to speak to a 4TB drive / Windows doesn't understand 4k Byte segments.
If this was originally from a different operating system, then your data is fine but can only be accessed by that OS (i.e. Windows doesn't understand Linux nor MAC File Systems).
If the original system this came out of is more 'modern', you may just need to patch Windows with the Drive Maker's files (go to the hardware maker's website and see for some patches) or you haven't applied Windows 7 SP1 yet (required) to understand 4k Byte Segments instead of the older 512k Byte way drives used to be stored.
If the drive crashed, and depending on the 'value' of the data on it, are you willing to spend a few THOUSAND dollars to recover it? There are Data Recovery Services out there, but they all have different rates and what your 'outcome' demands determine the costs (for example you expect to get the drive back with all the data on it in perfect working condition as compared to just a bunch of DVDs of recovered pictures and music files only and the 4TB tossed in the trash).
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