You'll rarely need a 4wd vehicle to get to any of the Washington ski areas. It doesn't hurt, but the roads will almost always be clear except for the last few miles, and even those last stretches are typically only a problem when it's dumping.
I do not agree with this statement entirely. It's true that you generally stay low (rain) until climbing the last few miles to any of the resorts. However, there are often chain requirements (which do not apply to 4WD vehicles) when it's dumping. Maybe when snow expectations are 10"+ or so.
You can find out pass reports/restrictions (Stevens, Snoqualmie) and access roads (Mt. Baker Hwy, Crystal Blvd) here.
http://www.wsdot.wa.gov/traffic/passes/camera.aspx
As an Easterner, I did not have experience with chains until the NW. I might just rent a 4WD to not worry. But it can be miserable if you have no experience - or no chains with your rental. (Note: I even had to chain up a 4WD once or twice a year). However, look at the forecast and decide.
If I were you, I'd visit Crystal on the Monday and Tuesday. Since it's almost exclusively a local mountain, the crowds are much smaller on the weekdays. In my experience, the crowds at Baker tend to be lighter overall.
I agree. However pre-Christmas, I do not think any of the mountains will be that busy...weekday or weekend. Lonely on weekdays and friendly on weekends. Doubt it would impact the experience too much. But if you want no friends on a powder day at Crystal....
Many of the Seattle skiers like my friend Larry Schick are lazy about doing the longer drive to Baker, especially when Crystal is good.
Seattle skiers are a spoiled bunch. Many north Seattle metro skiers will only go to Stevens, while South metro will only ski Crystal. People get picky. Very few go to Baker @ 2.5 hrs away. Also, I had friends who would not ski until the base gets to 100"+. A little insane.
Alpental has got some great terrain, but you need to watch the snow levels--the base elevation is only 3200 feet, so it's often sticky at the bottom.
I guess Alpental has not even opened yet. Given this (and the lazy/cost-saving attitude of The Summit management), I would avoid it. They probably have not done a lot of control work and are slow to open terrain. Also, the biggest bottleneck in WA state skiing is the Edelweiss chair -- an aging double to the summit. Weekend liftlines were always 15 min. Alpental looks to only be operating on weekends pre-Christmas too.
You might want to check out Stevens instead. It has the best intermediate cruising in the state (Crystal can be too easy/too steep for some people. Baker not long enough). There are some nice expert pockets too - 7th Heaven, Double Diamond and Mill Valley (Backside). Most of this is open. Also, the snow quality is probably the best in the state; there is often a pressure gradient funneling cold air from Eastern WA through the pass keeping the snow cool and dry.
If you have 4 days, you should probably spend 2 at Crystal since the terrain is extensive and worthwhile. Great place! If you wanted to minimize driving, take the other 2 at Baker. Mix in a day at Stevens if you wanted to see more areas.