First thing is to answer the question of where the 4/3 designation comes from. This refers to the sensor size, but is not the sensor dimension in any direct way. The sensor is the size that a sensor would be in an obsolete video camera tube with a diameter of 4/3 inches. This was some kind of known standard at some point in the somewhat distant past and somehow is now being applied to digital camera sensors. Makes little sense to me or anyone else, but that is how it is and you shouldn't feel bad for being confused.

The actual sensor usually has an active imaging area 17.3 by 13.0 mm. The diagonal dimension (roughly 22 mm) is the only thing actually fixed by the "standard", but most cameras have a coincidental 4:3 aspect ratio. A Canon APS-C sensor is about 40 percent bigger (22.2 by 14.8 mm) and has a 3:2 aspect ratio.

A 4/3 camera is a digital SLR. A micro 4/3 camera has the same sensor, but no mirror (and hence is not a digital SLR, but is much smaller).

Olympus PEN E-PL3 $600 (with kit lens) 12 Mpx and PEN E-P3 $900 (with lens) 12 Mpx

Panasonic Lumix G-1 Panasonic Lumix GX-1 $540 (body only) 16 Mpx.

A cadillac MFT (micro four thirds) Camera is the Olympus OM-D M-E5. Price tag (body only) is $1000. It is made to look like a film camera, so you are paying for the retro looks. 16 megapixels. Made to compete with the Sony NEX-7, but without the bigger sensor.

Similar to these micro 4/3 cameras is the Sony NEX-7. This is what is called a "rangefinder style" compact camera. It has an APS-C sensor (crop factor 1.5, sensor size is 23.5 x 15.6mm - a tad bigger than the APS-C sensor in a Canon DSLR). This camera weighs 10 ounces (body only) and has been promoted for ultralight backpacking. It does support raw capture. It captures 24 megapixels, which I frankly think is too much for several reasons. The captured files are too big, and cramming this many pixels on a sensor of this size sacrifices image quality. Price is about $1200 for the body only.