4/3 was never successful - only ever achieving low single digit market share. 4/3 cameras and lenses were not really any smaller or cheaper than aps-c even though they had a smaller sensor. It was a good move by panasonic and olympus to move to m4/3. m4/3 will basically kill off 4/3. I don't think you'll see another 4/3 camera from Panasonic, and I think only one or two more Olympus 4/3 (basically ones that were already designed and nearing production - if they aren't killed off before release).
The Japanese makers pay a lot of attention to their home market and if you look at 2009 sales in Japan - Panasonic is in 3rd place (behind Canon and Nikon) in 2009, with their m4/3 models. One issue I have with m4/3 at the moment is their high price (they get away with it for now since they're the only game in town) - look at the features they provide as compared to entry level DSLRs and it doesn't justify their price (the ep-2 is twice the price of the Pentax Kx !). But with the success of Panasonic, you'll see a lot more competition in the mirrorless interchangeable lens camera market - with Samsung, Sony and Nikon to bring in their models this year - which is good for consumers (Canon is surprisingly quiet, they might end up being last to this market, or they've kept the leaks at bay).