The need for all four engines being operable isn't changed by the viability of 3-engine flight. Once the plane lands its grounded for revenue service, and even three engine ferry flights are a huge affair to pull off.

So, de facto, each engine is strictly required, and having twice as many as a twin, quad jets are half as reliable as a twin, all other things being equal.

(me: licensed dispatcher who worked with the BAE-146 for a time. Incidentally, "BAe" stands for "Bring Another Engine" <grin>.)