What exactly does it mean that the system is 4M in debt?

Does it mean that if everyone tries to close their CPDs, there will not be enough ETH go give out?

If the number of DAI in circulation should roughly represent the amount of ETH in collateral, does it mean that there are 4M DAI not covered by ETH, shouldn’t that make the supply of DAI is too high and the price of DAI should go down? (which is not happening)

You are correct. Even though there is an additional 5M dai unbacked in the system we are still above peg due to high demand. I see this as a positive sign actually. Personally I would like to see flop auctions postponed so that the additional 5M being accumulated for the debt auction can be put towards restoring peg and shoring up the system in case of additional downwards market pressure.

Might be helpful read: