I definitely wouldn't include the third and fourth attacks in such a feat. Doing so would effectively put three levels worth of class features in a single feat, and it would allow other martial classes to easily replicate one of the Fighter's few unique features.
As for the baseline extra attack, I think it's worth considering who the biggest beneficiaries would be. Fighters, Paladins, Barbarians, Rangers and Monks already have the ability, of course. Bards, Warlocks and Wizards can get it as a subclass ability, but it's worth considering whether it would be useful to any other subclasses. Sorcerers could potentially benefit from making two attacks after casting a quickened spell, but I a cantrip would likely be preferable in most cases. Clerics with weapon focused subclasses could potentially benefit more, but the attacks would still be competing against spellcasting. I think the biggest winners, though, would be the Rogue and the Druid. Rogues are likely to be making weapon attacks every round already, so an extra attack is pure upside, and a Moon Druid could double up on some powerful attacks in beast form.