How many bytes long should nSequence be?
The sequence number is always 4 bytes, it is a fixed size, little endian, 32 bit integer which makes it always 4 bytes.

What you are missing is the scriptSig. The scriptSig always begins with the size of the script (as a compact size unsigned integer). If the scriptSig is empty, then the size must still be there to say that it is a 0 length byte. So the first byte of your "sequence" is actually the length byte for the scriptSig that says it is of length 0.