Here is my input on it, and I am not faithful to either to be honest.
When I was into dirtbikes, my ride was a Yamaha YZ 426F. First off from my experience was that these older yz motors were pretty damn durable. The main maintenance that had to be performed was an oil change every 10-15 hours (less if racing), Checking the valves, and timing chain replacement due to stretching. The main thing the bike also liked to eat was the chain and sprockets. I kept them properly tight but they still were eaten none the less.
In comparison at the time when these bikes first came out, the CRF 450 was known to require more maintenance. The early bikes required the usual valve checking, but also the pistons needed to be replaced more frequently. I believe in the newer motors (talking 05+) they became more bullet proof from what I've heard.
Anyway back to the 426... The power was always there I must say. The torque curve was broad and the engine just responded at any rpm. The engine did have a hit, but it was different than the 2 stroke. There was no feeling like when a 2 stroke gets on the pipe, but you could feel when you got into the meat of the power.
Another difference is engine braking, where unlike a 2 stroke, when you come off the throttle of the 426 the engine actually would slow the bike like a diesel truck. Many people who came off 2 strokes never cared for this because if you let off the throttle on a 2x the engine just seems to coast (even though thats bad to do due low amounts of oil coming in). But me, I liked it because instead you would just roll the throttle on and off and had that control and felt I was faster through some areas on the 426 than I was on a 250 2 stroke because of it.
In the end 2 strokes are cheaper to maintain when it comes to rebuilds due to less parts. The maintenance is also MUCH faster. But to me is it cheaper overall? that's arguable because I feel that for the most part they balance out, unless you trash the motor. If the motor has done something like dropped a valve than it is cheaper to fix a 2 stroke than a 4 stroke just due to the lower number of parts that need attention.
In the end to me, I like both honestly, which is why I have my regular 250R's, and also have that hybrid I'm working on that uses the 426 motor.
But one final thing I will say, I was never impressed with the 450 quad motors... without major attention they just never felt strong like that 426. I know its a different thing being a bike and not a quad but still... they just don't have that feel.
