What to do with 4 days in Lisbon area and 4 days in Porto area?

It's down to your personal interests/preferences which are not stated. It's entirely imangineable that someone could spend a week each in Lisbon and Porto and not feel sated, and another person could spend a day in each and have had enough. As tiny as Portugal is, it is culturally rich and scenically breathtaking and we found the people refreshingly open and friendly.

Again it depends on personal preferences but IMO there isn't really a compelling need for a guide. We toured the entire country with just the RS guidebook. Rented a car after our stay in Lisbon and returned it up in Porto before staying there. The roads are well-maintained, and iPhone GPS functioned perfectly. Most locals 40 and under speak excellent English (mandated in school), and over-40 speak at least some. And many older folks also speak French as it was previously mandated in school (a plus to me as I'm bilingual). Plus Google Translate app is always a fallback.

I'd say just do some research on those areas and see what piques your personal interest the most. Whatever you do you'll want to return to Portugal again 🤗

I agree it all depends on your interests. We spent 5 nights in Lisbon, including a daytrip to Sintra and a brief stop in Cascais. We didn't see everything that interests us in Lisbon. Do you like museums? We spent a lot of time in the Gulbenkian. Spent a day in Belem visiting the Jeronimos Monastery, the Discoveries Monument, had lunch, and the Coach Museum. Spend a day visiting the Sao Jorge Castle and then explored the Alfama District. Stopped at the miradouros, had drinks, and enjoyed the views. We didn't have time for the Tile Museum and a few other museums. Does our sightseeing interests align with yours? If so, I would do only one day trip from Lisbon and that would be Sintra and Cascais.

Not as into museums as you but thanks for the ideas ;
We like to cover ground :)

Have been doing lots of research; it’s confusing

Given your limited time, if you want to visit Tomar and/or Fatima, I would do it on the way from Lisbon to Porto instead of a day trip with backtracking. I visited Tomar by train from Lisbon, stayed one night, and then on to Porto. With a car you could probably visit both Tomar and Fatima, and either spend a night or maybe continue to Porto at the end of the day, given that you say you like to move on quickly. I found Tomar very interesting and quite different from Lisbon or Porto. For Duoro Valley, I did a small group van tour, daytrip only, with Cooltour Oporto. I booked directly with this tour operator rather than going through a middleman like Viator.

We are thinking of doing a Douro Valley tour from Porto -- a van tour or a boat tour. What did you like about the van tour and who did you book with?

I was there in 2019. My van tour was with Cooltour Oporto. It included 2 Quintas with tastings, lunch at a restaurant, and a one hour boat ride. I easily get bored on boatrides so a longer boat tour did not appeal to me. I liked the small group tour, and that we visited smaller family run vineyards.

At least 1 Lisbon day will be a day trip to Sintra and Cascais, of course…

That 1 day is going to be doing a lot of work…

Sintra is overflowing with good stuff. I think many people struggle to do Sinatra (and nothing else) in one day as a day-trip from Lisbon. In addition to having plenty of eye candy, these days Sintra has plenty of tourists too, so expect some crowds there.

Trying to do Sintra + anything else in a single day (and starting and ending in Lisbon) will be a pretty busy day. Just saying’

While you are in Porto I would suggest a drive out to the Arouca Suspension Bridge. It is a pedestrian bridge in an absolutely stunning setting. It was deserted when we walked over it in September. You do need to purchase tickets for the visit. If you are into nature it is a must see. It was the world's longest pedestrian suspension bridge when it opened in 2021.

I just returned from Portugal having visited many of the places mentioned in this thread. Please know that Portugal is "hilly". When doing a walking tour it seems that everything is up hill...both ways. And most of the historical sites have LOTs of steps.

Just my opinion, but I was not impressed with Sintra. But then we have seen alot of European churches, monestaries, castles, etc. After awhile they are all the same.

I strarted my trip research looking for guides for those places we wanted to visit. It was a failure and a waste of time.

Viator and other sources do a package deal that may or may not suit your goal. Hence, we just did our own thing.

One of your "own thing" was to purchase a pass on the Yellow bus Hop-on-Hop bus system in Lisbon. It includes a river cruise on the Tagus, and access to subway, bus, and train. We used all but the subway. With a local city map and a bit of flexibility most people can use it effectively.

We spent five days in Lisbon and three in Porto. We also spent two nights at a winery in Peso da Rèuga in the Douro Valley (10 min taxi ride from train station). Very nice place that also served a nice breakfast and dinner. We did a one-day tour to two small wineries with lunch at the second one (Bruno Cardoso was very good and grew up in the area). There are numerous trains to/from Porto to Rèuga, which is a nice ride along the Douro River. Links below.

We visited Fatima but it was not a full day. The next day we took a tour of Batalha, Tomar, Alcobaca with Sara Cruz, Go! Walk Portugal Tour. She was excellent and these monasteries are not crowded. You could skip the Jeronimos monastery in Lisbon (which is really crowded) and see more at the other 3 monasteries.

https://www.casaldoscapelinhos.pt/?lang=2

https://www.tripadvisor.com/Attraction_Review-g1028273-d17594998-Reviews-or10-Boro7_Authentic_Douro_Valley_Tours-Peso_Da_Regua_Vila_Real_District_Northe.html

https://www.gowalksportugal.com/tours-around-coimbra/tomar-batalha-and-alcobaca-world-heritage-of-central-portugal