1) 250F falls beteen the 250 and 125
2) 450 is more powerful than a 250, and not THAT far off a 500
3) 250F in the "lites" class, which used to be the 125 class.
4) 450 in th MX or SX class, which used to be the 250 class.
5) A 250F won't "beat" a 250. A 450 runs with 250's and generally has more HP and torque. There aren't any 250's left in Pro Motocross, or 125's for that matter.
Here is the skinny without getting in real deep:
1) The 250f is less powerful than a 250 2-stroke but it is more powerful (higher peak horsepower) than a 125 2 stroke with a broader power curve.
2) 450F is similar in power to a 250 2 -stroke but as with the 250F and 125 it has a higher peak horsepower with a broader power band.
3) 250F bikes race against 125 two strokes.
4) 450F bikes race against 250cc 2 strokes or they can be raced in the open class against 500cc or any size bike for that matter, heck a 125 can be raced in the open class under most sanctioning bodies. But as far as classes go for your purposes they race against 250 2 strokes.
5) A 450 is going to compete more closely with the 250 2-strokes that is the class they were designed to compete in.
All of that being said I have turned nearly identicle lap times on a track with a 250f as I did with my 250 2 stroke and was not far off on the 125 either, so track layout plays a big part in this as well. The four strokes have greater torque than their 2 stroke class counterparts and therefore can be ridden in a higher gear on most tracks and will require less shifting to attack the track at race pace. There is alot more that goes into these comparisons, but that is the quick and dirty.