Fifth Engine for the White Knight II?

I was just thinking it might be wise to put a fifth small turbofan,
something like a Williams International, above the pylon, just in case.

I'm assuming this thing can just about glide anywhere, but I'm wondering
how well this thing flies with multiple engine outs. Since they will
presumably have paying passengers in this thing, a small pusher fan
might be in order. Those main engines seem pretty powerful for that.

Anybody have any idea how something like this might handle with a single
engine, and almost empty on fuel?