BS = Block size = 4 KiB
You missed the 12 direct pointers in your calculation
(12*BS = 48 KiB) + 4 MiB + 4 GiB + 4 TiB
For ext4, we use 32bit indexing for the extents:
2^32 * (BS = 2^12) = 2^44 = 16 TiB
which is the number you would e.g. also find on Wikipedia for the
maximum ext4 file size.