4 day trip to Portugal

4 Day trip? That is not enough time for Lisbon and Porto. You can do Lisbon and Sintra in four days. You can do Porto and surrounding area in four days. You can't do both. It will take half a day to travel between the two, be it on a fast train, plane, or car. 
If you go to Porto, I would recommend you stay across the river in Vila Nova de Gaia. If you stay on the hill overlooking porto, the sunsets and view are spectacular and the heart of porto is a short walk across the bridge or a short train ride.
As for Lisbon, it is VERY hilly. Sort of like San Francisco. An insider trick is to use the subway stations to get from top and bottom of hills. You don't actually get on the subway. You just use the escalators inside the station. One entrance will be at the bottom of the hill and the other will be at the top of the hill. Baixa-Chiado and Rossio Railway Station are two stations that definitely come to mind.
The subway is great in Lisbon but if you are coming from the airport during rush hour, luggage can be an issue. You might be better off getting an uber, which works in Lisbon.
We did a sunset cruise on a sailing catamaran in Lisbon and really enjoyed that. There is a really nice restaurant called Latitude 38 we really like and always go to. The food is authentic.

If you rent a car then I would say that it's doable, but you need a little speed. Make your visit to Sintra on the way from Lisbon to Porto. Sintra is small and in my opinion not worth more than half a day anyway. Lisbon and Sintra are places where you can easily spend several days though. But the drive between the two cisites is nice. So you can combine the two places if you like. In Lisbon you should concentrate around the city center. This is an area that is best explored by foot, as you have almost no places to park your car in the city. a few places that I like to eat in Lisbon are The Decadente Restaurante & Bar and Cervejaria Ramiro. The first place serves really good meat and the second really good seafood. In Porto I would recommend that you try the local fast food called Francesinha. Super unhealthy, but really yum. I have a few photos of it on this link. It's a supartoast with several types of meat soaked in beer sauce. https://travellingclaus.com/francesinha/

I haven't been to Porto. I've been to Lisbon, we had six nights there. We stayed in Bairro Alto, in a small B&B called A Casa das Janelas Com Vista. Highly recommend. 
We did Sintra as a day trip from Lisbon. There is no need to go on an organised tour, it's a simple train journey. When you get to Sintra, there are circular buses that visit all the main palaces. We managed to visit three out of of the five. There's a lot to see there, make an early start.
We also visited Belém as a day trip. Again it's an easy journey - we took the tram. The Jerónimos Monastery and Torre de Belém are both very worth visiting, and make time for a visit to the most famous café in town, Pastéis de Belém. There's a huge monument called Padrão dos Descobrimentos - it was enclosed in scaffolding when we visited in 2017, but I imagine that will be all gone now. It has a viewing platform with panoramic views. In Lisbon itself there is much to see. We certainly only scratched the surface. We visited Castelo de São Jorge (worth paying extra to visit the Camera Obscura, but only if the weather is good). We had one rainy day when we were there, and we visited the Museu Calouste Gulbenkian. I can honestly say it's one of the best museums I have ever visited. We also enjoyed the Museu Nacional do Azulejo. We enjoyed spending some time just wandering the streets, especially in Alfama. It's also nice to walk around Alfama in the evening, when it's possible to enjoy hearing snatches of Fado, the traditional music of Lisbon. The Elevador de Santa Justa is worth visiting for the amazing views. There are several beautiful viewpoints in Lisbon, great for photos. 
We took a train to Cascais. The receptionist at the B&B booked a couple of pushbikes for us, and we cycled along the cycle route to Praia do Guincho, which was lovely. About 20km round trip. Finally - a lot of websites recommend a trip on Tram 28. We did that trip, you get great views of the city. When we were there, it was very busy. In particular, the return tram was absolutely packed. I'm guessing that the pandemic effect will mean the trams are currently far less crowded. However - be aware, we knew all about the risk from pick pockets in Lisbon and especially on tram 28, and despite that knowledge, my husband still had his wallet taken. These people know their business. They can have your valuables in no time, and without you knowing a thing about it. Take precautions, keep your valuables securely under your clothing would be my recommendation. My husband had a bum bag under his shirt, but they managed to get into it without him realising. Next time, we would use neck purses concealed right under our clothing. Pickpockets are a thing in most big cities, but Lisbon does seem to have a particularly severe problem. Fair warning! Despite this, we would still visit Lisbon again, it's an amazing city. Enjoy your trip, I hope this info is helpful.