My 4 year old dd is in private language immersion school in the US. She started last year in the 3 year old program. She was getting very bored at home and was asking a lot about going to school. We chose this school equally for the language immersion as for its academics (it goes through 12th grade). I read a lot about immersion schools before sending her and I saw many references that said that language immersion schools tend to have great academic programs. The one dd attends seems to.
We are a bilingual family at home (we use OPAL - I speak English and her father speaks Armenian). We also lived abroad when dd was first starting to talk a lot, so this is actually her 4th language exposure. She's very curious about languages already, trying to figure them out. She does speak Spanish (her target language... the school also has French) at school, but they said that we shouldn't expect her to be fluent until the end of this year. She sings a lot in Spanish at home, though. If I ask her a direct question, such as "How do you say..." she'll usually tell me, but is sometimes reluctant to. She is more likely to say something to me in Armenian than Spanish. I guess she just has it in her head that Spanish is for school, not home. They've told us that she'll get over that eventually (but it really doesn't matter because I don't speak Spanish).
I cannot say enough good things about immersion programs that I've seen so far. I've not seen her slow down in her learning because of the extra languages. She prefers English over all of them and prefers we read to her in English. Over the summer, she started sight-reading English words and has started writing them too if I help her with sounding out. She can't read or write with any other languages other than English, but has the colors, shapes, letter names, counting/enumerating, and vocabulary in them, so I think she's coming along fine.
Her school days are 100% in Spanish, btw. Oh, and most big cities have some sort of private, if not public, immersion offering. Try searching using the suggested links.
Good luck!
ETA: I did want to mention that dd's school tends to push academics. For a lot of people, that would be a turn-off. It fits for us, but I thought I'd mention it. I've heard other private LI schools tend to push academics, too. Don't know about public ones.