Wait a minute. Ninja? Samurai? Since when did 'western medieval fantasy' start to include elements from the Feudal Asia?
Since when was D&D only western medieval fantasy?
True, true. There's no line in the 4.0 Rulebooks that specifically state that D&D has to be western medieval fantasy. After having played through a couple of seemingly random adventures (because we often get sidetracked from the goal, when running long Herioc-to-Epic-Tier adventures), I've actually found that it is possible to bend the specs for certain weapons or items, to fit the setting better.
For instance, if it's a Three Kingdoms-era based setting, most generals would probably be equipped with large heavy-blades (Broadsword from AV2) and Halberds, while a few might be armed with Longbows, or for a Feudal Japan-setting, all the swords can easily have their types renamed (from "longsword" to "katana" or from "short sword" to "wakizashi") and nothing else needs to be changed. Hell, the individual player can even have a special modified version of a weapon. A rogue or assassin could easily get away with having an Assassin's Creed-style Hidden Blade as their weapon-of-choice, rather than a regular dagger, and it wouldn't impact the game as much as it would make the characters more separate from their classes' archetypical builds.
In the end, it really only comes down to the players' imagination. Besides, D&D worlds are designed to let whatever would look awesome have higher priority than common sense or real-world-physics.