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.
If we chose 3.99 billion, less 10 million than 4 billion, while 10 million will be current Bitcoin circulation，while NXT just less 3 thousand than 1 billion. So I don't think we can still call NEM issued 4 billion. Ten million difference should not be ignored. we will call the number 3.99 billion as 39亿(亿=1/10 billion) 9千万(千万=10 million) in China.Of course ,in west country ,3.99 billion maybe still regarded same as 4 billion. But our purpose is to happy asian people for this.So if 3.99billion can make it ,we will be OK.