LilianG - completely agree and it's exactly what research shows. I recently went to a session on multilingualism in children, with a panel of various language specialists and researchers.
You summarised what I learned there perfectly - little children are pragmatic and would only learn (use) non-dominant language if there is a particular communication need -ie someone who they can't speak to w/o using that extra language. Once that need disappears, so would the language.
Later on, from about 10 (maybe younger for some kids) - they can consiously decide that they want to learn another language. And with some effort, they can get to be quite goodie they keep it up.
So people at the session were also asking the same question: 'our young kids minds is so open for languages now, shouldn't we start teaching him Chinese (or some other difficult language) as they would learn it easily now'
And the answer was - 'sure, you can, but not with one weekly session. You'll need a native speaker who would have regular ongoing contact with the child. You should find Chinese, etc. playgroups, and get books and cartoons, and create a language environment around the kids...'
I am raising two bi-lingual Dds, and maintaining a non-dominant second language is daily work.
Learning few MFL songs with a language teacher at 4 is next to useless, unless you create some other reason for your child to speak that language.
Latin at 4,... is there really a reason to comment on that... Lol