"The government spends some $ 4000 million a year.........."
Why not to say $ 4 billion?
I am not a teacher.
"Billion" in the US unequivocally means 1,000,000,000. In an international setting, though, "billion" is ambiguous. In British English, it can mean 1,000,000,000,000. If you say "thousand million" or "million million", you will be understood by everybody.
It may have been used to achieve greater precision. Even in contexts where 'billion' is accepted as meaning 'a thousand million' - as it is, increasingly in Br Eng (especially in economic and scientific contexts) '4 billion' means 'more than 3.5 billion and less than 4.4 billion' - I think (my maths is long past its sell-by date). A politician might use this to allow himself some statistical 'wiggle room'.
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I think it could simply be that it sounds bigger in some way- it's a longer number to say and could carry more intonation if spoken or take up more reading time, so it could simply be used for effect.
PS I think that the British billion has pretty much disappeared- whenever I see the number I think of 1,000 million. In fact, the only times I ever see it mentioned are on discussions about it- I never see it used, though there may still be somee specialised areas that use it.