Having seen the one surviving episode of "The Celestial Toymaker," I'm not sure that the absence of the rest of it is such a great loss. At least, if we could only get a finite number of episodes back, I wouldn't want to waste one of the slots on that.
That's sort of why the question came to mind: in view of Underwater Menace 3, the general reaction to getting part 2 back was 'It's great they found something, but why couldn't it been from something else?' But everyone who's seen it says it's much better than part three... which is pretty much what people who saw it all back in 67 were saying all along.
So, the rest of Toymaker might be better than part four, and maybe even make four seem a bit better in context (bright happy atmosphere in part one darkening as it goes on), though I personally doubt it.
And of course, Toymaker was regarded as a classic... when it was all missing. That's sort of why having something from every story appeals to me: finding one episode can destroy a story's reputation (Toymaker), or boost it even higher (Evil of the Daleks).