It's hilarious concentration is being thought of a resource, when its whole reason for being is to limit the number of ongoing effects spell casters can have active.

That said I do like the idea of trading the ability to concentrate for another attunement slot. Martials should have more of those, so it plays nicely in to it. I would phrase it as part of attuning that 4th item any character forfeits their ability to concentrate on a spell until they break attunements to the point they are back down to 3 (or whatever other limit they have). So if you are using your concentration for attunement it's not even an option to cast a concentration spell (and forfeiting the attunement in the process).