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.