I started my first winter with a nice ski/alpine hiking jacket, the <a href="
http://marmot.com/products/details/zion-jacket">Marmot Zion</a>. It is awesome, 110% water proof and windproof (and soft, not a hard shell). It however offered no padding, and minimum insulation. If you're looking for a good ski type parka, also check out snowboard jackets as they can provide some more warmth and padding, although perhaps at the expense of style... ymmv. Either way, make sure it's seriously 100% H20 proof or you've wasted your $$$.
But, back to the Marmot, it was great till black ice took me for a quick slide (<10mph going round a corner) and ripped the elbow a bit. Also bruised my arm, but low speed so nothing serious. Scooter was fine too aside from a scrape on the center stand. I still use the jacket in torrential rains as nothing is more waterproof, but for most riding year round I've moved to the <a href="
http://www.revzilla.com/motorcycle/revi ... et">Rev'it Opera</a> jacket. It is awesome, does well in cold, wind and rain, and looks nice enough to wear out to non-riding events and the office. Oh, and it's padded. One thing I'll say, that is a disappointment, has been the button sewing quality. An easy fix for sure, but stronger thread and better stitching for the buttons should have been done for a >$300.00 jacket.
Anyway, I guess, what I'm getting at, is I feel better, safer, and more comfortable in the Rev'it compared to the ski jacket. I often do my base layer (wife beater), work shirt (polo or button up), and a hoodie in the extreme cold, and the Opera (with liner) in the winter. Come March/April I took out the liner and road the jacket till summer when none was used. It's actually a good summer rain shell, and I can deal with the warmth for my short ride in that situation.
I too have a 4 mile each way commute, Somerville ->Cambridge, and try and ride every day unless it's actively snowing.
Good luck with whatever you choose, also I'm on the hunt for those grip cover things in the above pic as my hands were iced no matter what gloves I used ($100 First Gears or $70 Perl Izumi bike gloves which were actually better than the FG's!).