You wouldn't say мальчики or мальчиков?
Numbers 2-4 take the genitive singular.
четыре have a declension in -a just like один becomes одна in front of a word that ends in - a ?
I learnt this word a long time ago (approximately 3 to 4 years?) and I still remember it!!!! And I haven't practiced Russian for a long time, too (I mean, I started to re-learn it a few months ago)