yes, that is an illegal formation. not enough defenders is the first display its illegal, there might be something with too many flank players, but its illegal either way. you can use a 4-3w-2n-1 to cover the flanks but still have strength in the middle. obviously pass through the middle
YOU're MANAGER, YOU MANAGE
Congratulation Arsenal!!
Hope these thread could help you, Formation-Tactic and GUIDE
I dropped into a friend's game who was running something like this. He was losing 5-0 about 20' in. His opponent was running 4 1 2 1 2. I don't think it got any better.
illegal formations will do that to you. that's why you should always make sure your formation is legal and not freak out when the opponent uses one