My husband speaks German with our children. He allows them to respond in a mix of German and English. (We live in the U.S.) I speak Spanish with my children. I used to make them respond to me in Spanish, but when my older one went to college, that started to slip with him. (It was a miracle to get him to return calls at all....) However, sometimes he does still talk to me in Spanish, especially if he's in a public place, talking on his cell phone, and would like a bit of privacy.
My younger son started to slip about two years ago when he had some school trauma related to his Tourette Syndrome. But he can still answer in Spanish when I insist, or if the thought he is thinking just happens to have some concepts he learned entirely in a Spanish-speaking context.
I read a Smithsonian article that said that English is a predatory language. I think that's true. My two children used to talk a mixture of 80% Spanish and 20% German that I called "Germish" with each other. But lately it's been 90% English. Sigh.
My first child mixed like crazy until around age 3 he started to sort things out. My second child hardly mixed at all.
When my older son was age 1 to 5, we were living in France, and he learned English at part-time family day care, and begin speaking French when he was 3. We moved to the U.S. when he was turning 5, and that was the end of French!
Based on this experience, I think that it would probably be possible for a child to understand well with more than four. But things get complicated. There are logistics to figure out, and there is stress for everyone involved. We kept a log book that went back and forth to day care with child #1, with his growing vocabulary, so the poor woman would stand a chance at understanding his mishmash!
We did it so that each parent would feel a close connection to the children. The ability to communicate with monolingual relatives is certainly a plus! (We sent child#1 to English-speaking day care because English is the common language my husband and I share.)
We do feel happy with the approach we took, i.e. it was worth the trouble! My husband says he has an easier time sharing his sense of humor with the children when he talks to them in his native language.
Edit to answer @CreationEdge question.
I didn't have that issue of the Smithsonian magazine handy so I checked the web. Here's a quote:
"Having achieved global domination, the English language ... is now used more by second language users (over one billion) than first( about 380 million).... English [is] a "predatory" language according to Catalan author and translator Teresa Solana.... According to sociologist Ruben Rumbaut, North America has proven for centuries to be a language graveyard where immigrant families' native language is almost always lost by the third generation." From Language of the Heart, Van der Berg
It might be easier to understand if we look at this example: in Mexico, there are still rural pockets where an indigenous language is still the primary language, but the number of speakers of these languages is shrinking as Spanish takes over. Yes, in that context, Spanish is a predatory language!