A rogue gets sneak attack die, or combat advantage if at the begining of their action they have cover, or are concealed.
The rogue can charge from behind a wall and get the sneak attack dice, but since odds are they won't really have cover unless there is a tapestry or something like that, most of the cases they actually have to move, and then charge. That move action removes their cover.
However, some skills allow you to move and attack in the same action, and those actions get sneak attack dice.
The general rule is, check for cover at the end of each action. (move, standard, or minor) If there is no cover at the end of each action, then there is no stealth on the next action.
Stealth: Free action made after moving
into a square at the end of an action.
You may use a move action to make a
Stealth check in your current square.
✦ Opposed Check: Stealth vs. passive
Perception. If multiple enemies are
present, your Stealth check is opposed
by each enemy’s passive Perception
check. If you moved more than 2
squares in the action prior to making
the check, you take a –5 penalty to
the Stealth check. If you ran, the
penalty is –10.
✦ Becoming Hidden: You can make a Stealth check against an
enemy only if you have superior cover
or total concealment against the enemy
or if you’re outside the enemy’s line
of sight. Outside combat, the DM can
allow you to make a Stealth check
against a distracted enemy, even if
you don’t have superior cover or total
concealment and aren’t outside the
enemy’s line of sight. The distracted
enemy might be focused on something in
a different direction, allowing you to
sneak up.
✦ Success: You are hidden,
which means you are silent and
invisible to the enemy (see
“Concealment” and “Targeting What You
Can’t See,” page 281).
✦ Remaining Hidden: You remain hidden as long as
you meet these requirements.
Keep Out of Sight: If you no longer have
any cover or concealment against an
enemy, you don’t remain hidden from
that enemy. You don’t need superior
cover, total concealment, or to stay
outside line of sight, but you do need
some degree of cover or concealment to
remain hidden. You can’t use another
creature as cover to remain hidden.
Keep Quiet: If you speak louder than a
whisper or otherwise draw attention to
yourself, you don’t remain hidden from
any enemy that can hear you.
Keep Still: If you move more than 2 squares
during an action, you must make a new
Stealth check with a –5 penalty. If
you run, the penalty is –10. If any
enemy’s passive Perception check beats
your check result, you don’t remain
hidden from that enemy. Don’t
Attack: If you attack, you don’t
remain hidden.
✦ Not Remaining Hidden:
If you take an action that causes you
not to remain hidden, you retain the
benefits of being hidden until you
resolve the action. You can’t become
hidden again at the end of that
action.
✦ Enemy Activity: An enemy can
try to find you on its turn. If an
enemy makes an active Perception check
and beats your Stealth check result
(don’t make a new check), you don’t
remain hidden from that enemy. Also,
if an enemy tries to enter your space,
you don’t remain hidden from that
enemy.
Best Answer
Unless there are other circumstances to keep you hidden, you will stop being hidden when you leave the cover provided by your fighter ally.
You could maybe use a lance or whip to attack through the fighter's square (these are the only reach weapons that I could find that are not "heavy", and so are usable by small characters without problem)
But...
It sounds like you're trying to Sneak Attack?
In which case, you don't need to be hidden.
And if your fighter ally is in melee with your target, you don't need advantage.
The rules for Sneak Attack say:
With the fighter in melee with your target, as long as you don't have disadvantage, you can either melee attack with a finesse weapon, or attack with a ranged weapon. No hiding required.