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The time scaling was exponential for each layer but I don't remember the numbers given. I know it was 5 minutes of reality is an hour of level 1 and I believe they said that the 10 hour plane ride would be decades on level 3 (but don't worry, they won't be there for the 10 hours).
But I don't believe an actual scale for Limbo was ever given. The brain problems I believe were tied to the fact that you'd be there a very long time in isolation, the space would possibly be unconstructed nothingness (total sensory deprivation) or whatever you made of it, and worse you quite possibly wouldn't even know that you were in limbo.
It wasn't explained in any detail how Cobb and Mal had made it to Limbo the first time just that they were playing with going deep. They were shown sleeping on their living room floor with no means of nourishment or care so they can't have expected to be under long, it just ended up being decades in Limbo. As for whether it is always Level 4 i'd guess it must be because Ariadne and Cobb certainly knew that was where they'd go when they went one level down from the Snow Fortress level (though why that level had dreaming equipment would be a mystery, unless she made it up out of whole cloth).
Yes, Limbo seems to be a shared space that exists even when nobody is in it (Fischer ended up in Cobb's constructed area of Limbo before Cobb was back there) and Saito constructed in own part of it. It isn't explained how Cobb got there, just that it was apparently arduous.
The really dangerous part of Limbo seems to be that you don't necessarily know that you're in Limbo and so aren't trying to get out. Cobb and Mal didn't know they were there and when Cobb eventually figured it out after decades from his point of view, he has to completely subvert Mal's subconscious to get her to leave. Saito didn't know where he was and it required Cobb finding him and reminding him. Cobb had been there before so it makes sense that perhaps he'd be psychically prepared for dealing with it.
I don't believe the movie ever makes it clear whether Fischer knew where he was. That leaves just Ariadne that had an easy time of it. It could be argued that this was a combination of having intentionally sent herself to Limbo and having Cobb there from the very beginning explaining where they were.
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