I'm not so sure about this. As long as you actively use all four languages on a regular basis there is little chance that it fades from memory. A language which you won't use will surely fade from memory.
I used to work in a quite international setting and used all four languages I knew on an almost daily basis, speaking with colleagues from various countries. Often on the spot changing from one language into another. However, after having left that position a couple of years ago I now notice that the fluency in at least 2 of those 4 languages has gone.
It is true about having to keep using them or they wither, you are left being able to grasp the idea of a conversation but not speak. Mind you have I have hardly used French which I was fluent in for 20 years and it still was like a bump in a rug to trip over for me speaking Spanish

It is painfully obvious talking to many Americans that the mind expanding effect of a 3rd (and more) language is absent. Many even often older people don't have a functional first language to think in, not using punctuation at all in written messages. At least they are easy to spot and ignore.