how about the elder 2 children together? Or put all 4 together? You could make the decision last minute, even after starting school, you have to 15th September (don't quote me on that date but whatever date it is, it's immovable) to change schools without prejudice.
Your youngest in maternelle will ironically probably benefit the least from Le Verseau, especially at over 4k per year! There's no reading or writing taught in maternelle, you would have to think, is there a benefit to the extra 3 or 4 hours a week in English at school, when your child has an English environment at home? Just my opinion. I believe le Verseau likes to keep it roughly 50/50 anglophones/francophones and 2 times when friends have phoned it has been one of the first questions, whether francophone or anglophone. With such a large number of anglophones in one school, there is a risk your children will take much longer to speak French and might even just refuse, as little children can suss out quickly that when others speak English, why speak French!!! Experts quote 3 factors crucial in acquiring a (second) language : consistency, frequency, need. So when there is a bilingual environment where a monolingual child joins, sometimes they work out that there is no need to learn the second language because everyone else understands their first language. I work in a bilingual class and there are a couple of children who do exactly this - the entire class speak English fluently as a first or second language, 1/3 speak French first language, 2/3 speak English as a first language, a couple of the English speakers are completely resistant to speaking French in the French time and it's an uphill struggle, they just don't want to speak French because they don't need to, they know the teacher speaks English too, they know the other children speak English, so why the heck, they think, should I speak French!!!!! It's definitely a minority reaction, but it happens. Le Verseau thankfully has most of its teaching time in French, only a few hours a week in English, but with 50% of the class speaking English, there is a risk your children might react the same way and simply refuse to speak French, even when they can. You see what I mean? In a monolingual school, there is no choice, it's either French or nothing.
Anyway, it might be an idea to look for a school with places for all 4 in Rixensart, especially at Ecole du Centre and Ecole Bourgeois which are reported in the articles as the 2 communal Rixensart schools which have lost the greatest number of children. They are the 2 communal schools nearest to the centre of Rixensart. There are 3 reasons given in the articles given for the reduction : 1 the move towards francophone parents sending their children to Dutch speaking schools in the next Dutch speaking commune, the asylum seeker children in the "passerelle" French second language classes at Ecole du Centre and thirdly the disruption caused by the building work around central Rixensart by the RER (express train network) where they are adding 2 extra rail tracks, meaning parents have got fed up with road closures!
Rixensart is lovely, so is the next town (also part of Rixensart commune) called Genval and parts of Genval are a bit more pricey, especially around the lake there. If several Rixensart schools have places, then I'd take all the places, also look at the same time for housing (I'm now guessing you've not moved yet) as the rental market is much smaller in this area than in Brussels where you look for a school first and then somewhere to live later, in Rixensart I'd do both at once, possibly looking for a house first actually.
I think you could do with our Genval mumsnetter on here to help you, she knows the area far better than me. I think her ID her is SAMiAM or something like that. Look back through the thread and you'll find her-send her a pm, I'm sure she'll read it.