From a thread on theRPGsite by "Windjammer", here are a series of links. Wizards of the Coast has released a huge amount of free material for 4th edition Dungeons & Dragons, available through a variety of links. All of the following are links to downloads from the Wizards of the Coast website.
The Monster Manual updates will get you a 40 page Monster Manual, replete with the original art work. As a quick and dirty rule of thumb to scale these monsters to your desired level: for each increase of one level, you add +8 hit points, +1 to: to-hit/AC/ saves/damage. If you want precision, print p. 7 of this document for the damage per level expressions. The document also contains the skill check table, which comes in handy for running the game.
All together amounts to over 160 pages of 4th edition material, all freely released by Wizards of the Coast. Basically, you could run the entire game out of these PDFs and the Rules Compendium (a $20 paperback), if you feel you need to be precise. An alternative is to download from third-party compilation of all WotC pre-release previews, which contain all the rules for skill challenges, ritual magic, traps, PC races, and so on.
Best of all, by doing this you are not restricted to only three levels of play; and you actually don't have to use pre-generated characters; both in marked contrast to the previous Playtest freebie. What is more, you have a game at your hands that only contains the "core experience" of the game. Not 15 classes, 30 classes, 500 feats, just the gist of it.