Not too sure why the "4/3 so it is slightly of lower quality".
However, it is a smaller sensor size.... So... there are differences, but I don't know if those differences are necessarily a bad thing. For example, Olympus has a x2 crop while the Sony has a x1.5, depth of field differences, etc...
Having just recently purchased an Olympus e510, I can tell you that there's quite a bit to read out there about what is available nowadays. Not having used any of the other cameras, I can't recount my experiences between cameras with larger sensors and the Olympus 4/3 system.
I can say though, that I have been very happy
with my Olympus purchase (picked up the 2 lens kit)... Having played around with film for years, the deciding factor for me was that I'd have a chance to play around with two lenses (both of which are quite good for kit lenses!). Reasonably priced, and knowing that it's just a matter of time before I'd dish out more money for flashes and additional lenses... I wanted to find something priced under $1000 that I could experiment with before I really dumped a bunch of $$$ into a system that I would be stuck with in coming years. For the price, ($650US) I felt I had a system comparable to most Nikons and Canons with a nice set of lenses to experiment with before having to make a decision about what system I'd go with.
I can say that so far, I haven't been disappointed with the Olympus and am definitely going to stay with this system.