So a thought occurred to me -- how hard would it be to do this in 3.5, and how different would it be? Here's the 3.5 version:

 

Goblin Knifethrower CR 4
Goblin; rogue 4
LE Small humanoid ( goblinoid )
Init +4; Senses Darkvision (60'), Listen +8, Spot +8,
Languages: Common, Goblin
AC: 18, touch 15, flat-footed 14;
Hit Points: 25 (4 HD)
Saves: Fort +3, Ref +8, Will +2
Speed 30 ft. (6 squares)
Melee dagger +2 (1d4 /19-20 )
Ranged dagger (thrown) +6 (1d4 /19-20 )
Face 5 ft. Reach 5 ft.
Base Atk:: +3; Grp -1

Abilities: Str 11, Dex 18, Con 14, Int 10, Wis 12, Cha 6
Special Abilities: Evasion (Ex), Sneak Attack +2d6, Sneak Attack [+2d6], Trap Sense (Ex) +1, Trapfinding, Uncanny Dodge (Dex bonus to AC)
Feats: Armor Proficiency (Light), Simple Weapon Proficiency
Skills: Balance +13, Hide +15, Jump +9, Listen +8, Move Silently +15, Sleight of Hand +11, Spot +8, Tumble +13,
Possessions: Goblin Throwing Knife (x5); masterwork studded leather (small);

Feat: Rebounding Attack
You can bounce a specially-designed knife off walls and other obstacles to flank your opponent.
Prerequisite: EWP (Goblin Throwing Knife)
Benefit: Whenever you make a ranged attack using a goblin throwing knife, your opponent is considered to be flanked if he is adjacent to a wall or any other object which would provide at least partial cover to anyone behind it.

Goblin Throwing Knife: 1d3 Slashing, Wt. 1/2 lb.

 

Some general thoughts:

This 'build' is much less useful in melee than the 4e build, but it looks pretty effective at range, especially if it's positioned behind partial cover (so it can shoot out and they can't shoot back), or is out of reach of the party's melee folks, who have fewer exotic movement options at level 4 than their 4e counterparts. 

 

I could have avoided the Feat and simply given the Knifethrower an (Ex) ability. This would make it more powerful, as it would not need to waste two feats and could have, say, Point Blank Shot and Rapid Shot, which, combined with the (Ex) ability, would greatly magnify its damage output (two ranged attacks per round at +7 doing 1d3+2d6 each if the target is against a wall). In retrospect, I probably should do that, especially since per 3e guidelines, one monster of CR4 is supposed to consume 1/4 of a 4 person party's resources. (Did anyone ever use that? I always ran teams of monsters and the players tended to win handily anyway.) The use of the Feat also opens it up to abuse, because someone, somewhere, will be playing a character who is a goblin, or was raised by goblins, or can spell goblin, and this feat makes it considerably easier to get flank. (Of course, there may already by a feat like this buried in the Complete Guide To Left-Handed Half-Elven Multiclassed Rogues).

 

So why did I use a feat, originally? Because I think one of the best innovations in 3e (over prior versions of D&D) was the "Monsters and players are built with the same rules". If "the rules describe the world", there's no reason a PC or NPC can't have this ability. The upside to it is that the world feels more "real"; the downside is, an ability which might be balanced for a monster can be very unbalancing when it's used by a PC. Consider that Polymorph allows the use of feats (and, for that matter, (Ex) attack abilities -- a rogue/wizard, or a rogue with potion, wand, or scroll, or whatever -- could pick this up. It's hardly the most unbalancing of abilities, but it's one more thing to consider. When you design a monster, you don't normally consider the ramifications of it also being a PC. The thing is, in Actual Play TM, the horror stories about polymorph and what-not rarely occur, but there's a stark and unrelenting terror of the infamous Character Optimization boards among some developers.