I can safely say I've only hit 4th plateau once, and even being an experienced DXM user (30-40 trips beforehand) I was a bit overwhelmed at certain points.
I took 50 robogels and then downed a bottle of vicks with just dxm, I was up at around 1.4g of dextromethorphan, which may have been a bit overkill but I was building a tolerance at that point.
By the time I really started coming up, the vicks was giving me some bad nausea because I always take gels instead of chugging syrup, and due to being so fucked in my head, I got worked up and started feeling really hot (I never did find out if I was just imagining it or if I was really running a temperature), and I turned a fan on, which helped. What didn't help was that the room I was in only had deep red led light strips running along the ceiling, and I was getting a real sorta hellish vibe from the room. My voice seemed deep and muffled and I couldn't understand what anybody was saying.
So I'm really hot, like burning up, peaking and tripping out that I'm now dying and I am halfway into hell, and my friend's dad comes in to check on me, knocking the fan out of the way. This caused me to feel twice as hot as before, and looking up at him babbling incoherently (he's a drunk) I saw him as the devil. Then my friend (bearded hippie type) makes him leave and points the fan back at me, and I look up at him and see him as Jesus due to the hippie look and cooling me down with the fan.
His dad, being drunk and just trying to help, keeps coming back in and knocking the fan out of the way, so every time he comes in, I get burning hot, and the room gets redder, and every time my friend forces him back out, the room gets a blue color to it and cools down.
At this point my trip is basically a crazy fucking movie, I am essentially watching Jesus and Satan battle for my life, Jesus (my friend) trying to drag me out of hell and trying to keep satan (his dad) out.
This went on for another hour until I faded out of consciousness. I don't even remember this little stretch. But when I came back, I threw up in a bucket, and the epic battle for my life ended, seemingly in a draw, leaving me in limbo, and I dragged myself onto the couch. From here on I just drifted from life to death in my head, debating on which was the better option at this point. Eventually, I came down enough to realize I had been tripping, and I just let myself fall asleep.
Sorry for going on so long, I know this isn't supposed to be a trip report, I just wanted to illustrate the ridiculousness that takes place at the 4th plateau. Anything is possible, and the most regular things can rise to epic proportions, for the better or worse.
Just remember the essential set and setting, because they will become a big part of a 4th plateau trip.