you could say that regardless of how decentralized bitcoin is or was, it always relies on miners (pools) to secure the blockchain, whether this is 4, 40, 400, 4000 etc.. regardless of the #, they could at any time decide to collude for 51% hashing power. There is no incentive for pools to conspire and regardless of the number of pools needed to reach 51%, there will always be a certain # of pools that "could" conspire. There are 4 US banks that could conspire to destroy the USD, but what incentive do they have to do so?
The largest banks clearly conspire, as an explicit or tacit cartel, to obtain maximum profit. That is one reason why bank fees are so high compared to the actual costs (including fraud damages) and large banks are so obscenely profitable.
The correctness of the bitcoin protocol apparently was based on the implicit assumption that a sufficiently large number of users would have sufficient hashpower to fight attacks, and, having a vested interest in the health of the network, would want to do so. (It is the same assumption used to justify democracy with frequent direct elections.) That assumption is not valid if there are 4 mining entities that hold the majority of the hashpower, and are not even significant holders of bitcoins.
Yes, but these are pools are made up of thousands of participants who can at any time redirect their hashing power. Pools are setup only for consistency of income.