It is harder than your average red. TBH I'd call it a black... Are you comfortable on the harder bits at the end of GT black? It's not quite as hard as that, but, the challenge level is fairly consistent, you don't get the breathing space that a trailcentre trail usually gives you of feature-rest-feature... That makes the tricky bits seem harder, over the course of a run.
But it can all be done wheels-on-ground and cautiously if you choose, and the nice thing about uplift is you don't need to rush into it, it's not like a trailcentre lap where you get one go at it... You can (probably should) take a gentle sighting lap, then repeat and improve...
ST4 should be bang on, I did it on my old Soul when it first opened, slowed things down but it all went just fine. Works better on a little bike than on a dh bike! I really like it... It's lost a nice section to a diversion currently, due to bad hanging trees, and it does peter out a bit before it gets to the bottom, so it's not perfect but it's a grand day out. I think if you had to ride to the top you might be quite critical in places, but you don't so **** it
Oh aye... The world cup route isn't massively harder (though it is more... intense, maybe? Even less respite). SO if you're enjoying the red and comfortable on it, you can mix in the downhill too. If you skip to 12:20 or so of this vid:
You can see a path that goes off to the right... Nip down there, over the stream and up the other side and you arrive at the wallride, it's relatively mellow from there down and a lot more fun on the wc than on the red, which has pretty much run its course by then. There's still a couple of big features but the woods are lovely.