200 to 250 (3.5) in a year is absolutely doable at 3-4 days a week of structured training. Probably 3 structured rides, and 1 free ride.
After that, it's anyone's guess.
Getting from 3.5 to 4.2 W/kg is, in my opinion, dependent on genetics and QUALITY of time put in.
It make take a year of dedicated and structured work to get that 3.5 up another 10% to 3.9 - just really depends on the person...or it might come easy for you. I feel like most people could probably hit 4 W/kg with the right training - but I also feel it could very well be the upper limits for a lot of people as well.