If you have a lot of layers, you can try the steamer to remove a layer or 2 at a time. You have to be conscious, though, that if you apply the steamer in 1 place for too long, you'll soften up the plaster and end up with more work than you want. You need to guage yourself with the steamer as you go. Try taking off a couple of layers at a time. After you finish going around the room once, you can try the steamer again to take off another layer or 2 that was left behind the 1st time. It doesn't matter if you take off 1, 2, 3, or all 4 layers in one shot. You just have to be conscious of the consequences of applying a steamer to a 'spot' on the wall for an extended period of time. If you're planning on putting up wallpaper again on the wall when you're finished, you don't have to be too neat. You basically have to re-plaster any gouges and paper the wall. If on the other hand you're taking the paper off for a paint job, you would want to be careful during the removal process because plastering even minor mistakes 'all over the place' will show when you paint. Remember, a plaster wall can look like a drywall job but there aren't that many tradesmen who can make a plaster wall look like a drywall wall. They get good $$$ but they earn it.
In sum, be careful if you steam. Do a little bit at a time. Go slow. If you end up with a layer or 2 because you're finding that the steamer is damaging the wall, you're back to trying my 'tips' in my previous follow-up. Good luck and if you get a minute, post a follow-up to let us know how you made out. A lot of us would like to hear an update.