And four-engine aircraft are designed to deal with the loss of any engine without any danger.
Two-engine aircraft are designed to deal with the loss of any engine without danger. My question is whether a four-engine aircraft that is missing one will still have the safety margin of being able to withstand the loss of any other engine without danger.
Listen to yourself! Are you not equally concerned with a two-engine aircraft losing another after losing one? Why not? Why do you consider it perfectly safe for a two-engine plane to lose one engine but not for a four-engine plane to lose one engine? You say "but what if it lost another"? Well, what if the two-engine plane lost another? That would concern me a heck of a lot more. But you simply dismiss it with "Two-engine aircraft are designed to deal with the loss of any engine without danger." But so are four-engine aircraft. You seem totally irrational.
To sum up your argument: it's a disaster for a four-engine plane to lose an engine becasue it might lose another and then it would only have two engines left even though it can still fly with two -- but that's a disaster. But for a two-engine plane to lose an engine, that's hunky-dory because they are "designed that way" and if they lose one they could never lose another even though that would leave them with NONE -- and that's perfectly safe.
On a two-engine plane, there would be no question but that the loss of a single engine would warrant landing the plane as quickly as safely practical. The question is whether the loss of a single engine on a four-engine plane should warrant an immediate landing, or if the plane should continue toward the planned destination. If the four-engine plane which is operating on only three-engines could safely withstand the loss of any other engine, then such a plane operating on three-engines would have about the same safety margin as a two-engine plane operating on both engines. If the loss of another engine could create a dangerous situation, however, then the plane would not have the same safety margin as a two-engine plane.