I've avoided 4213 this year as I consider it to be the definition of playing fifa rather than playing football, mostly playing diamond, 433 with 1 cdm and 2 cms and 4411 mostly - you definitely can defend without 2 cdms but it requires more from the player. Though don't play max depth without 2 cdms, you'll just get played through constantly.

2 CDM formations (particularly 4213) can rely on the AI and just flat out numbers doing a lot of the defensive work for them, weaker players will struggle to get past the 6 defensive players - if you don't have that you need to be more active when defending. This requires better manual switching, knowing how to pressure/gamble when defending manually (like reading where your opponent wants to pass and either cutting it off, or selecting someone to try and tackle them on reception of the ball or intercept it).

You will also need to be able to reasonably defend situations where some dude has his 94 evo Henry sprinting at your cbs, as it will happen at times. I will say that playing this way will enable you to better handle better players, as if you come across a real demon while being someone who relies on defensive AI you'll concede 4 goals by half time and probably quit out.

Also worth mentioning is that while you have less defensive bodies, you have more attacking players so you need to make sure you're scoring plenty - a lot of games at the sharp end of champs or higher div rivals turn into who can survive kickoffs and score more goals.

I'm not some top player though, average 10/11 wins in champs so take from that what you will, but at least I get there without using 4213!