Ive been through these same thoughts as you and my size is very similar to yours but I do a whole lot more racing. All in various types of terrain, Desert to woods.
I own a Yamaha WR 250f (Wifes bike) its uncorked JD jet kit etc. etc. So I know this motor well. It is good! Stone cold reliable. The valves don't tighten up like many small bores. I do ride it sometimes for a whole day when my bike has been set up and ready for a race the next day.
But I'm here to tell you that IMHO after riding a 2 stroke 250 for many years you will probably be searching for more juice, even with a 290 kit. Get in deep sand or get up in altitude or start climbing steep grades its gonna zap the poop right out of that little motor. I know your an Oregon resident so I would say the 450 is gonna feel large and heavy even if your not dropping it in the slower more technical terrain that we experience up here in the North West, Ive rode them.The power and the fuel injection of the 450 is phenomenal but the bike feels like a mass under you. I would also venture to say the little Yamie 4 banger is not gonna feel lighter than what you already have. My wifes Yamaha certainly don't and I kind of doubt the Gas Gas chassis is lighter than the aluminum framed Yamaha.
So what would you gain by getting the four stroke. More noise, more maintenance, more possible catastrophic failure if it did go south, and then more money dumped into it to fix it.
Oh I forgot, you won't have to mix fuel anymore. Not really such a benefit is it. Some would claim more tractability but a well set up Gas Gas 300 is about as close as you get to 4 stroke traction.
If it were me I would trade the 250 in for a 300.
It's so much easier to be lazy on a get away with it than the 250 2 Stroke. Next season the 300's should have electric start for us tired older fellows. The 300 is really the closest thing to a 4 stoke without taking that plunge. Its still light and flickable enough when needed, very tractable and I need more power will never enter your mind.
Good luck on your decision. Only you can decide whats right for you. Just try to ride as many bikes that you are thinking your interested in before you buy and get some feed back from the owners about that particular machine.
Roscoe