From the DMG/Compendium, the important part for your question:
Forced Movement and Terrain
Difficult Terrain: Forced movement
isn’t hindered by difficult terrain.
Blocking Terrain: Forced movement
can’t move a creature through blocking
terrain (page 61). Every square along
the path must be a space the creature
could normally occupy.
Challenging Terrain: Forced movement
can make some powers more effective or
hinder them, depending on the specific
challenging terrain. The DM can
require the target of forced movement
to make a check as if it were moving
voluntarily across the terrain, with
the same consequence for failure.
Hindering Terrain: Forced movement can
force targets into hindering terrain.
Targets forced into hindering terrain
receive a saving throw immediately
before entering the unsafe square they
are forced into. Success leaves the
target prone at the edge of the square
before entering the unsafe square. If
the power that forced the target to
move allows the creature that used the
power to follow the target into the
square that the target would have
left, the creature can’t enter the
square where the target has fallen
prone. If forced movement pushes a
Large or larger creature over an edge,
the creature doesn’t fall until its
entire space is over the edge. On the
creature’s next turn, it must either
move to a space it can occupy or use a
move action to squeeze into the
smaller space at the edge of the
A DM can allow a power that pushes a
target more than 1 square to carry the
target completely over hindering
Published in Dungeon Master's Guide.
And from the DnD FAQ ( http://wizards.custhelp.com/cgi-bin/wizards.cfg/php/enduser/std_adp.php?p_faqid=1396 ):
Are zones that deal damage (like the Wizard power Stinking Cloud) considered ‘hindering terrain’? Can I make a save to fall prone and avoid being forced into one?
No, zones are not considered hindering terrain. Hindering terrain refers to more permanent features like pits, cliffs or pools of lava.
The resultant answer: For zones, base DnD rules do not give a saving throw to avoid being put into the fire. For pools of acid, you are given a saving throw to catch yourself before it and fall prone.
1. Yes, Just like push/pull/slide someone over a cliff, the same applies here. If it is a beach or river bank then they get wet. If it is a dock or cliff then they fall (save if applicable), then get wet. See below.
1a. That would be up to the creature and any other affects on the creature, but I would say they fly. See below.
2. No, and Yes See below.
3. No, see below.
4. Yes (sort of), see below.
Rules from DDI
Slide: PHB1 pg285; MM1 pg283 - When you slide a creature, there’s no restriction on the direction you can move it.
Pull: PHB1 pg285; MM1 pg282 - When you pull a creature, each square you move it must bring it nearer to you.
Push: PHB1 pg285; MM1 pg282 - When you push a creature, each square you move it must place it farther away from you.
Forced Movement: PHB1 pg285; MM1 pg311
Movement that a creature is compelled to do, specifically a pull, a push, or a slide. A creature can be moved in other ways, such as through teleportation, but only pulls, pushes, and slides are technically forced movement.
Two-Dimensional: Forced movement is normally two-dimensional; all the squares of the movement must be on the same horizontal plane. Forced movement can become three-dimensional when the target is flying, is moved through a substance such as water, or is on a non-horizontal surface, such as an incline, that supports it. This means an earthbound target cannot normally be pushed to a square in the air, but a hovering target can be. Similarly, a target can be pulled down a flight of stairs, and it can be slid in any direction underwater.
Currently update to wall of fire notes that:
In the past, this was ruled differently, see the edit history of this answer for previous interpretations.
At least now it's a lot simpler. Now it doesn't matter how large the creature is nor how many squares it moves, it takes damage once per turn. Still, if multiple PCs are playing ping-pong with a mob, they will indeed take damage every PC's turn that they enter the wall.