I am making the same decision, I have a 1985 kx-125 and a 2009 YZ-250F that I put a 290 kit in the 250f had a $5 ring dicintigrate and pretty much destroyed the motor, matter of fact I purchased a replacement 2008 motor for it. Rode the kx 3 seasons and alot more than planned due to the 4 stroke having issues. The cost of rebuilding it (KX) is peanuts compared to the 4 stroke, parts alone for me on the 250f would have been over 3k to fix mine right, let alone labor!!
I will look at the Kaw and Yamy 4 strokes but I am leaning towards a yamy or KTM 2 stroke to replace the 250f, which I have for sale right now. Another thing im looking at is the weight of these bikes, a 450 feels like a tank to me, I had one did not like it at all, they are really heavy feeling. No wonder all these guys get hurt on these 4 strokes, they arent light like a 125 or 250 2 stroke. One of them clobbers you in a crash its gonna hurt!! Anyone looking for a really nice takenn care of 250 f lowered by 1'' or so let me know