Jester David
Hero
The rules have that covered already.For a two armed being a feat is required to cast a spell whilst you have a weapon and shield (or 2H or off hand weapon) equipped. If you are going to give out the equivalent of free feats, you should at least mention it in the rules.
And duel wielding whilst benefiting from a shield?
Using a two handed weapon whilst benefitting from a shield?
If you want to allow it, you need to mention it. If you want to disallow it you still need to mention it. The rules as written are for two-armed characters.
But even if someone does have the feat that lets them cast spells while holding a shield or weapon, they can't cast a spell and attack at the same time. They don't get extra actions.
They can hold four weapons, but that doesn't mean they get extra bonus actions for offhand attacks.
Having four able arms just means they have more chooses for which weapon makes their offhand attack. And means they always have a hand free for casting spells. They don't need to state every benefit of having extra hands, they just need to say "you have extra hands".
(Also, that's the least benefit of War Caster, which primarily grants advantage on Concentration saves and the ability to cast spells as an opportunity attack. So it's not like they remotely get a feat for free.)