quiltjoey,
We have a Singer 4622A that is very similar to yours. Paid all of 3.75 for it at a yard sale, then bought a part and fixed it.
It is a very decent machine for an early to mid 90s machine. When you oil it pay very close attention to the moving parts behind all the sheet metal under the face plate. Ours was very sluggish on the ZZ and patterns until I got the Tri-Flow in the right place. Since then it's been sewing very nicely. We don't use it much, but it does work and gives us no trouble.
The yellowing is from light exposure. Many of the machines made in the 80s 90s era have badly yellowed plastic parts from sitting in the sun or under lights. It won't hurt anything, it's just not pretty.
As the others have said, if you're happy with it, it was a good buy.
Joe