For winter and early spring and class 4 it's hard to beat western Oregon.
Where I live in the southern part of the state (or for that matter the whole state) A combination of runs from just above sea level to 3,000', pacific storms, rain, snowmelt, and spring water add up to wide variety of runs to choose from during those times (I average 10 or more days per month of boating during the winter). If you make it out here let me know.
some of my favorites include (in case you want to investigate);NF Smith in Ca. (fantastic scenery and rapids, I've run it in January, sunny and close to 70 degrees many times), Illinois (great overnighter), Lake Creek (best play run in Oregon), Opal Creek (my vote for best class 4 creek in the state)