While we're on the topic, what exactly is 4/3 and why is Olympus championing this format? Is it merely an aspect ratio? What are the supposed advantages?
4/3 is a format of roughly 13x17.3mm, with the obscure name 4/3" coming from the weird jargon of video formats, where it referred to a sensor of total size 13.5x18mm. (The actual image size is smaller since as with all sensors, some pixels around the edge are not used as part of the output image). It also happens to have a 4:3 aspect ratio, as all the traditional video formats did.
It is the format adopted by Olympus (and Panasonic) for DLSR's aimed at roughly the same market as Canon's EF (now 14.8x22.2mm: it has got slightly smaller over the years!), Nikon's DX (about 15.8x23.6mm), and the formats used by Fuji, Pentax and Sony (all the same as Nikon's DX).
I can only guess as to why Olympus chose this format while other SLR makers chose slightly larger ones, and my guess is that the others were interested in a good degree of backward compatibility with their existing lenses for 35mm film cameras, at least during the transition while new lenses in the new formats (EF-S, DX, etc.) were added, but Olympus had little use for such backward compatibility, since it had no auto-focus 35mm film lens system. It had only a rather old fashioned, manual focus, all mechanically coupled lens mount system, of little interest to most DSLR buyers.
So Olympus decided to start from scratch with a new format smaller than 35mm (like everyone else) but also a completely new lens mount designed for that format (rather than reusing an "oversized" 35mm format lens mount like everyone else) and a completely new lens system. Since the format size was chosen with no regard for backward compatibility, it is rather natural that it ended up a bit different from, and smaller than, the EF-S and DX choices: backward compatibility probably pulled the EF-S and DX formats up closer to the 24x36mm of 35mm. Nikon might have had a goal of doing everything with DX format, before going with 24x36mm for one high end model, and such a goal would have lead to choosing a larger format than needed only for the DSLR mainstream.
Olympus has I think indicated a goal of matching or exceeding the technical quality of 35mm film with the smallest feasible lenses and cameras, helped by changes like having the lens mount closer to the sensor than was needed with 35mm film SLR's which allowing smaller, lighter bodies.
For the size and weigh advantages, look at the E-410 (435g, including battery) and its kit lenses, the 14-42/3.5-5.6 (190g) and 40-150/4-5.6 (220g). Still bigger than most digital cameras (since most are fixed-lens digicams in far smaller formats) but smaller and lighter than other DSLRs and lenses covering the same FOV ranges.