Bitlocker won't initialize on a 4TB drive with 4K sectors

A MUCH easier solution. After formatting the drive, copy some data to it before turning on BitLocker. I did this for a WD 4TB drive and now everything is working fine. I copied a folder that is under 2GB with about 650 files and now BitLocker is running.

Manufacturers will commonly state a formatted capacity, but due to the different way operating systems interpret byte sizes, and the amount of physical sectors on a drive, this value can vary. The true size of a drive is dependent on the number of physical sectors (and the size of these sectors).

For example, a Western Digital WD4001FAEX has 7,814,037,168 sectors, each which is 512 bytes in size. Multiplying the number of sectors by the size per sector, we obtain:

7,814,037,168 sectors * 512 bytes/sector = 4.00078703 x 10^12 bytes

Using base 1000 units (e.g. 1000 bytes = 1 kB), this would be converted to megabytes as:

4.00078703 x 10^12 bytes / (1000x1000 MB/byte)  ~= 4,000,787 MB

However, Windows interprets disk sizes in bases of 1024 and not 1000 (e.g. 1 kB/kilobyte is not interpreted as 1000 bytes in Windows, it is interpreted as 1 kibibyte/KiB or 1024 bytes) on Windows, and thus the maximum size Windows would report (ignoring any filesystem overhead) in mebibytes is:

4.00078703 x 10^12 bytes / (1024x1024 MiB/byte) ~= 3,815,448 MiB

This is very close to your reported formatted capacity (3,815,174 MiB). It's likely the remaining ~300 MiB is consumed by the filesystem itself, which is also likely formatted as NTFS. NTFS is a journaled filesystem which requires additional information to be stored (like the master file table, and several other hidden metafiles).