What did Paul believe about 1 Corinthians 10:4 with regards to the teaching about bread from heaven, spiritual food?

Paul understood "bread' to mean revelation from God meant to build faith through suffering and being rescued.

To be Christian is to suffer. Jesus said believers needed to pick up their crosses every day.

To understand the reasoning, we must study what the offer from God to believers is. It's not avoiding hell and going to heaven. Salvation is partnering with God to complete creation, subduing the earth. A car that does not transport its passengers to their destination is called dead. A power drill which does not switch on is also called dead. People who can't perform the function for which they are created, to subdue the earth, are dead. The result of the Fall.

God called His promise to Abraham the promise of making his children blessings to the world. This was by making a people who would lead the world out of selfish living, which is harmful in the long run. God incubated this group in Egypt, not in Canaan, where they originally lived, so that they could see the world system at its worst, making them the bottom of the food chain, the lowest in its hierarchy, slaves. They were motivated to leave, but the promise of a better situation was not definite. Leaving the world system was risky, by all reasonable thinking. What if the alternative was worse?

To motivate Israel to come out of Egypt, God empowered Moses to perform signs, so that the nation could be assured that the future God had planned for them would indeed be better. God continued to perform signs for Israel even after she left Egypt. This was by making her suffer food and water shortages, and then saving her from these sufferings. In doing so, God was preparing Israel to enter the Promised Land, rest, destination, which was to become blessings to the world. In the rest, she would undergo suffering and be rescued, everyday. Pick up a cross and be raised, everyday. This would motivate the world to come out from selfish living and serve God,by living selflessly, when they saw it was not risky, was possible. The whole of creation waited in eager expectation for the full revelation of the sons of God. Romans 8:19. Thus the earth would be subdued. God's original plan.

But the plan did not work, and God rejected Israel:

1 Corinthians 10:1 I do not want you to be unaware, brothers, that our forefathers were all under the cloud, and that they all passed through the sea. 2They were all baptized into Moses in the cloud and in the sea. 3They all ate the same spiritual food 4and drank the same spiritual drink; for they drank from the spiritual rock that accompanied them, and that rock was Christ. 5Nevertheless, God was not pleased with most of them, for they were struck down in the wilderness.

And kept rejecting her until His real people, identified not by being descendants of Abraham by flesh, but by faith, emerged.

Luke 2:14"Glory to God in the highest, And on earth peace among men with whom He is pleased."

This was always God's plan, to have a people, who were not identified by flesh, but by faith.

Baptism would make these people (who knew God was not received by struggling to meet His expectations of selflessness by human effort but by faith, those called born of water, like Noah and faithful Israel born out of the water that cut off the world) experience cross and resurrection, suffering and rescue, picturised by the synechdoche "Christ", in the phrase "they drank from the Rock, and the Rock was Christ”, to motivate still more people to come out of selfish living.

Suffering leads to God's great works being manifest, and prepares His People to be blessings to the world, when they do not forget the great works, but bury the revelation in their hearts, so that it takes root, and produces more spiritual grain, more living water.

To answer your question, those with whom God is not pleased are those who are not suffering, are not living the life God intended for those who want to be His followers, because the meaning of the Gospel offer has been lost because of time. Salvation is not going to heaven, it is living a meaningful life.

All Scripture from the NASB.