Surely he took up our pain and bore our suffering, yet we considered him punished by God, stricken by him, and afflicted.
But what about the word "our" in verse 4? Isaiah was speaking to the people of Israel, right?
Right, that's the narrow local context. Let's see the broader context, Isaiah 53:l
1 Who has believed our message
and to whom has the arm of the Lord been revealed?
Here there is a hint that the message is from the local Israelites to the Gentiles.
Zoom out a bit more,
1 Listen to me, you islands; hear this, you distant nations: Before I was born the LORD called me; from my mother’s womb he has spoken my name.
The message was for Jews and Gentiles.
But are all humans included in that word "our" in Isaiah 53:4?
Yes. This was how Peter understood it as well, 1 Peter 2:
24 “He himself bore our sins” in his body on the cross, so that we might die to sins and live for righteousness; “by his wounds you have been healed.”
i.e., Jews and Gentiles
25 For “you were like sheep going astray,” but now you have returned to the Shepherd and Overseer of your souls.
Does Isaiah 53:4 really apply to humans that disregard the existence of Elohim?
The message of Isaiah 53:4 is to be delivered to all people. After having heard the message, if they do not believe the message, then the content of the message does not necessarily take effect on the unbelieving hearers.