There is some understandable confusion about what squares
creatures are or are not allowed to stand in. For a full run-down, see the
compendium’s “occupied square” entry, but here is a brief summary. Because 4e
is an exception-based rules system, it can get messy. However, the general rule
is:
You cannot occupy a
square that is already occupied by another creature or by an object.
This rule has numerous exceptions:
If the creature is an ally , you can stand in
their square if they are prone . If the creature is an enemy , you can stand in
their square if they are helpless If the creature is an enemy , you can MOVE through their square if they are two sizes
smaller or larger than you (but cannot stand in the same square as them at the end of your movement). If you are a tiny creature, you can
occupy the squares of larger creatures (regardless of ally or enemy). If you are mounted
on a larger creature, or a smaller creature is mounted on you, in which case Mounted Combat rules apply instead.
One exception that does NOT exist is that you cannot occupy an ally’s square if that ally
is two sizes larger than you (unless you are tiny or mounted on them). Yep.
Read over those exceptions again: you can walk through an ally's space; you can walk through a much larger enemy’s space, but you CANNOT end your move in either of those spaces. This rule means that you
cannot occupy a square occupied by your huge ally (unless tiny or prone), but it also means that you can’t slide enemies into the same
squares, even if one is much larger, because they are allies to each other and
that’s against RAW (unless one of them is tiny or prone).
This is an oft-confused concept because you would think that that would be one of the
exceptions, but it is not. It’s sort of like saying “This is always true,
except in cases A, B, D, E, and F.” You might think that case C is an exception
as well, but alas it is not.
Good Hunting!