I don't think there's ever been a double-bird-strike incident, though. And what dual engine failures I can think of are due to failure of a shared system (e.g. fuel exhaustion, c.f. Gimli Glider).

[Edit: yeah, yeah, forgot the Tom Hanks movie, sue me. I do wish folks would respond to the much more important point below, which isn't invalidated by a single data point though.]

Constructing solutions for multiple-mode failures like this is a bad engineering smell. Almost always the solution isn't actually helping anything, and often makes things worse in whatever metric you're looking at. In the example here, having four engines makes the chances of total thrust loss lower, but it doubles the chance of a single engine failure. And the literature is filled with incidents of theoretically-survivable single engine failures that led to hull loss as a proximate cause (generally by confusing or panicking the crew).