Husky had a 4 stroke 450 in 1983.
ATK started 4 stroke 550's then.
KTM was working on one, didn't come out by '83.
There was a fourstroke class Pro race at IndianDunes, but those bikes weren't proven for moto and the bikes cost well over $3,500 a fortune to me, since I was getting Suzuki's and Kawasaki's for about 800 bucks, brand new.
Only real 4 stroke I've ridden is the blue YZF, maybe 426, I don't remember.
I took up windsurfing in late '83, and haven't looked back, except to keep track of my mates from the '80's.