But you are not trusting hashrate to secure bitcoin, you are trusting the top 4 mining pool operators. 
If the pool operators collude...
Why won't the pool participants just leave the pool and join some other pool?
Sure, they can do that, but the damage will already be done.
* Currently only 10 major pools to choose from.*
The Powers that could collude 4, could collude all 10.
That is the problem when a coin is not really decentralized.
Pooling would have to be banned to return security to the hashrate and not the pool operators.