If anyone is interested in reading defenses of Plantinga's argument from different perspectives, you can do so by paying a visit to this question.

• P1 (God is omnibenevolent) is probably uncontested. Pretty much everyone concedes this as an axiom in the definition of God.
• P2 (God is omnibenevolent => God is not capable of moral evil) should be uncontroversial as well. God cannot do evil. It's impossible/unfeasible for Him. It just won't happen.
• P3 (God has free will) is based on the intuition that if humans (and angels) have free will, it would be very strange for God not to have free will as well. One could reject this premise and claim that, perhaps, God is a deterministic being who created free creatures. Sure, one could hold such a a view, but it would be a very novel (and strange) one, wouldn't it?
• P4 (God has free will => God is capable of moral evil) is based on the same intuition used by the free-will defense against the problem of evil. If evil is explained as an undesired price of having creatures with free will (which God was willing to pay because of how valuable free will is), then what the defense is basically saying is that free will => capable of moral evil. So P4 is just a particular application of that rule to God, if we concede that God has free will.

• P1 (God is omnibenevolent) is probably uncontested. Pretty much everyone concedes this as an axiom in the definition of God.
• P2 (God is omnibenevolent => God is not capable of moral evil) should be uncontroversial as well. God cannot do evil. It's impossible/unfeasible for Him. It just won't happen.
• P3 (God has free will) is based on the intuition that if humans (and angels) have free will, it would be very strange for God not to have free will as well. One could reject this premise and claim that, perhaps, God is a deterministic being who created free creatures. Sure, one could hold such a view, but it would be a very novel (and strange) one, wouldn't it?
• P4 (God has free will => God is capable of moral evil) is based on the same intuition used by the free-will defense against the problem of evil. If evil is explained as an undesired price of having creatures with free will (which God was willing to pay because of how valuable free will is), then what the defense is basically saying is that free will => capable of moral evil. So P4 is just a particular application of that rule to God, if we concede that God has free will.