Just wondered if this was widespread, as it seems common with children I come across. Lots of children like to try out English with me (bilingual kids where neither language is English). We know lots of other bilingual children, though not many where one of the languages is English. DS really wants to pick up a few words of each language, generally hello, goodbye and the numbers. He finds French hilarious for some reason.
I know plenty of tri and quadri lingual DC. I am not convinced that any age is "right" for the introduction of another language - I think that providing the right type of instruction is more important that a particular age when it comes to language learning.
Having said that, in my DD's case, she had mastered reading and writing in French and English before starting Spanish and she is making cracking progress in Spanish, which is taught through immersion techniques.
Four is tough. Actually, three is usually quite tough though some combinations (eg French/Italian/English, French/Spanish/English) are seemingly much easier to manage than others (French/Farsi/Mandarin, French/Russian/English).
DS(4) speaks English/ Dutch and is interested in French and German. He's always asking what things are called in the other languages. Obviously that's not quite the same as learning them yet.
I knew two boys who had a German father and Russian mother. The parents practiced OPOL but spoke French together as that was where they had met and were living. When the boys were toddlers they moved through work to the Netherlands where the boys went to nursery, and then school. They were trilingual in Russian, German and Dutch and understood French perfectly although I never heard them speak it. By the time I lost touch the boys were also speaking excellent English having learnt it at school. I have no idea which language was their 'first' as I heard them speaking together in all of them.