A good system typically has - in order of flow - a sediment filter to remove suspended solids, one or two carbon filters for capturing mostly organic compounds (that's the difference between a 4-, and 5-stage unit), the RO membrane itself, then an additional carbon "polishing" filter. (I'm not convinced of the benefit of a 5-stage unit, and in the three years I've been selling systems and RO components, have never sold a single 5-stager.)
As to the waste water - I prefer to call it "flush" water (doesn't sound so wasteful) - something in the range of 4:1 or 5:1 is "standard". However, there are definite ways to improve upon that.
As Rick pointed out, there are pumps available (about $200) to push that flush water back into your hot water supply, so it gets used. (It goes into the hot water after your heater, so the extra minerals don't precipitate.)
Another alternative - about 1/3rd the cost and one I recommend to anyone - is a so-called "permeate pump". Added onto any RO system (up to about a 100 gpd rated capacity), it uses the flush water to pressurize the membrane, producing water at a faster rate and greatly reducing the amount of flush water generated.
I added one to the BBRO ("Bare-Bones") system I use for refilling fish tanks, and it cut the refill time to somewhere between a half and a third of it's former performance, and the flush to the sink is a slow stream rather than its former "jet" of water. I recently received an email from a customer in Arizona who added one to a 60 gpd system, telling me that the flush stream volume is less than 20% of what it was. (Neither of those is very scientific, and are probably nothing more than "eye-ball estimates", but the improvement is significant!)
