A 4 eye check is typically used as quality control in processes such as banking where disbursement of funds is checked twice or approval of disbursement is done after a 4 eye check.
This is an NVA In the process and in some cases can become an essential NVA (regulatory requirements) but does not add any value to the customer or the process. The recent Citibank/Revlon issue has clearly shown that the 4 eye check is definitely not fool proof to say the least!
what is needed; are systems and processes in place that eliminate the need for a 4 eye ( a blind re-key as an example though also an NVA will improve accuracy and reduce costs). The right logic, validations and systemic integrity checks can do away with most such checks.