3,999,....  or 3,,888,...  to avoid 4 billion is just a trick of number.
They are (roughly/about) 4 billion if we round up.
The amount of NXT is 999,997,096. but people say/think NXT amount is 1 Billion.
Even NEM dev team said 4 billion NEM is determined by 4 times of 1 Billion NXT.