My wife and I are planning a trip to Seattle in early December for 4 days (minus travel time). What are some highlights that we should not miss? I want to drive up to Vancouver but I am afraid there will not be enough time. Are the roads going to be icy?

The drive to Vancouver is extremely unlikely to be icy. I think three to four hours for the drive plus the border crossing would be a reasonable expectation, so long as you do not proceed through the worst rush hour traffic.

Plan on the weather being rainy,so bring an umbrella and warm clothes, if it's not then consider yourself lucky. We have had a fabulous summer for being the northwest but the winters can be quite wet. The drive to vancouver should take you about 3 hours. Another option would be to take Amtrak from Seattle to Vancouver, it's a great trip.You could also catch the High speed Victoria clipper up the Victoria on Vancouver Island.
Take in Pike Place Market, everything downtown will be decorated for xmas.
Go to Seattle center and take a trip up the space needle.

If you have a car, the Bellevue Botanical Gardens do a wonderful outdoor Christmas scene that's worth seeing. Also, if the weather permists, make sure you get out to some of the fabulous sites nearby--checkout guideboods to see what you might like. Maybe even take a ferryride to Banbridge or one of the other islands for a daytrip. Also, if you like ballet, get tickets for The Nutcracker at the beautiful new Opera House--this company is among the top in the country and with the Maurice Sendak sets, it's a treat. If you like jazz, go to Jazz Alley one night (food isn't the best, so maybe go to a late show and have dinner someplace else first.)

Really, with only 4 days, there's so much to see and do around Seattle, I don't know if it makes much sense to drive to Vancouver.

See what's playing at Seattle Rep. We just returned from Seattle and saw "Topdog/Underdog". It was riveting and worth every penny. We are big fans of the theater's artistic director as she used to be that for Berkeley Rep. Our loss was Seattle's immense gain.
Do not miss lunch at Matt's at the Market.

If you want to spend a day in Vancouver, use the train. It departs Seattle at 7:45 AM, arrives around 11:30, then leaves at 6 PM and gets back to Seattle around 10. The northbound route will be gorgeous, even if the weather's poor; the return will be at night (obviously) but so would the car trip. Six or seven hours isn't a lot of time in Vancouver, but it's enough time for lunch in Chinatown or the West End, hit the aquarium or take a spin through Stanley Park, browse the shops along Robson, Yaletown, Granville Market, etc., then head back to the station for a relaxing dinner on the train. Much easier than driving, $45 bucks a head RT, and you won't have to cool your heels (or stress your kidneys, if you get my meaning) at the border in awful Christmas-shopping lineups. Use taxis in Vancouver (plentiful and cheap) and you'll have a great day.

Another vote for the train to Vancouver. Much of the route is directly on the beach in a very scenic area that most visitors never see as much of the coast there is somewhat away from the main roads.

Ken