In May I spent four nights in Lisbon (day trip to Sintra), two nights in Coimbra, and three in Porto. I stopped in Tomar off the train on the way up to Coimbra, and I did a day trip to Nazare from Coimbra by bus.
In retrospect one night in Coimbra was more than enough for me, really. I wish I had instead taken a night in Nazare away from Coimbra. But, you're right - December may not be a great time to visit Nazare.
Coimbra is really small. The highlight is the university at the top of the hill the town is set on. But that can be explored with a two hour tour. There are some Roman ruins outside of town somewhere, but I didn't bother with them. Coimbra is delightfully untouristy compared to Porto and (especially) Lisbon, though in December I suppose you won't find nearly as many tourists anywhere in Portugal. Otherwise, personally, I can't imagine spending four nights there but only four in Lisbon, which is so much bigger and has so much more to see. One night was enough for me for sure.
Porto is larger than Coimbra but much smaller than Lisbon. I am not a fan of port wine either (don't drink) but was going to explore the Douro Vally one day by train, but it rained so I didn't bother. Porto is charming especially by the waterfront, but unlike Lisbon there aren't many museums etc. to entertain yourself on a rainy day. I was meeting friends on my last day - otherwise I would seriously have considered cutting my last day in Porto and going back to Madrid (where I was flying out of) a day early. I don't think it's a mistake to visit Porto, but I don't think you need much time there especially if you don't like port wine.
If I were you, I'd try to start the trip up north (train directly to say Porto after flying into Lisbon) and work your way back to Lisbon if you are flying in/out of there, to avoid wasting the final night in Lisbon by training back. Adding an extra night in Lisbon wouldn't be a mistake in my opinion. FYI, Sintra has a lot of sightseeing opportunities, but it's hard to fit them all into one day, and in December your days will be short, so you'll have to plan carefully to fit in what you want to see.
