Who are the big 4 custodians?

Can you please elaborate on who exactly are the so-called 'big 4 custodians' in the world of cryptocurrency and finance? Are these companies well-known and reputable in the industry? What sets them apart from other custodians, and what kind of services do they typically provide to their clients? I'm particularly interested in understanding their role in safeguarding digital assets and ensuring the security of transactions within the cryptocurrency ecosystem.