May be anecdotal but my family is friends with two parents, the father French and the mother American. They and their children lived a few years in France and Montréal, and they spoke the two languages at home. I remember two of their children (around 8-9 years old at the time), not being really fluent in any language, but actually quite mediocre at both. They would start a sentence in french, struggle on grammar or vocabulary, and end up giving up and finishing it in English. I can't imagine what it would be like with two more languages.