They should buy EVERYTHING that cannot borrow money at 4%, because AMZN can borrow at 4%. The difference in the rates is free money economics.
You do understand that Amazon's borrowing rate reflects their risk. And that if they bought companies with a higher borrowing rate, which reflects their higher risk, then Amazon's overall risk level would go up to reflect that. And then Amazon could no longer borrow at 4%. There are very few free lunches in economics, this isn't one of them. A free lunch would be a private university issuing tax free bonds at below market rates to reflect the tax free status of the bond, while at the same time investing a chunk of their endowment in higher interest taxable bonds and not paying tax on that interest because they're a non-profit. Good thing that never happens!
That is what you say, but the FOMC is a private institution and if they want to bail out and lend at 0%, which they did to the automakers and banks regardless of their risks, they will.
It's not what I say, it's how finance works. If you don't grasp the difference between the Federal Reserve (I presume you realize the Federal Open Market Committee isn't an entity and were referring to the Fed?) and a corporation, it's really not possible to have an intelligent conversation with you on the subject.