Bike options

if you want more bottom end power, your either going to have to get a 250 two stroke, or a 4 stroke.
125's don't have bottom end.
But just remember that with more power, comes extra weight.

Keep the 125 for another year. Get really good at riding it, get stronger, smarter and mature... Then get a 250f... You will be much faster in the long run.

Yes the 150 does.

My kid has that same bike, we bought it about a month ago. I actually raced it in one of my classes last race and had a blast on it. I'm 5'8, 156lbs, B rider. What mods do you have on the bike currently?

Mine had an FMF shorty silencer, I actually liked the FMF Q that happened to come with it better because it gave it a nice long pull. Our track here is very hard/slick, I'll probably go back to the shorty on a groomed track. Mine also has Boyesen Carbon reeds and 50T rear sprocket (stock is 49)

Overall it does feel like it has a dead spot right before the midrange hit, I grew up on 125s but this still seems like a big dead spot. Very nice mid/top and you have to stay in that range if you want it to move. Some of the things I'm looking at:

V Force reeds are also supposed to boost power throughout the range.
FMF pipe for a little more low/mid (mine is still stock)
A big bore kit would get you more bottom, but it's not going to turn it into a 250.

Right now I'm being careful, because I find the spot annoying but what I love about the bike is that it actually has a pretty broad mid/top end power spread, and I don't want to do anything that would cost me any range. You already have to shift a lot on a 125, no need to make it worse.

I personally don't think it's worth the trouble to but a newer one just to go to the aluminum frame, other than the fact you can put '15 plastic on anything 05+ and it will look new. Some people prefer steel. Everyone prefers SSS forks to the '04 YZ forks, but I have heard with a revalve they can be pretty good. A lot of people also hunt on ebay for a set of SSS off of a later model, and the good news is that they should bolt on to your 04 without having to switch out the triple clamps.

If you are seriously racing, you're probably going to end up on a 250F unfortunately. The awesome part about staying with the 125 for as long as possible is that 1) it will teach you good habits as far as momentum and 2) you can run the dog crap out of a 125 2T with minimal expense and maintenance, compared to a 250F. This may or may not be an issue depending on your financial situation and how often you get to ride anyway.

So I guess the big question -- are you an occasional racer, serious racer, play rider, etc? Those things make a big difference as far as to which choice I would recommend.

My race vid if you want to see it: https://www.youtube.com/watch?v=8m5hX0YQnrY

Can't go wrong with that one!

^this. Stay on the 125 and beat the people racing you on bigger bikes. Then test ride a 250f or 250 smoker and get one depending on your preference.