I'm glad you had an amazing time in Funchal. βΊοΈ
The crab cake you had for dinner looks yummy, definitely more appetising than Royal's. One of these days I will get to try the real deal!
With four hours between flights in Lisbon you actually could make it into the centre, since it's only a couple of stops on the metro. You wouldn't have to go through customs or immigration at that point since you're arriving from Barcelona on an intra-Schengen flight. However, since you're not familiar with the city, you certainly wouldn't be able to do it justice and you'd likely be looking anxiously at your watch the whole time instead of enjoying yourselves.
Lisbon airport is really not too bad for a longish layover. Assuming you don't have lounge access, there's plenty of food and shopping to be done.
Our favourite place to eat at Lisbon Airport is called Cockpit.
As you can see from the picture, they have a small seating area where they do table service. If that's full, you can order at the counter and sit out in the main food court area.
Here's the menu. This is the Portuguese one but they do have it in English too.
Our favourite dish there is the "prego com presunto e ovo estrelado", which is a rustic bread roll with a beef steak, dry-cured ham and a fried egg inside. It comes with rustic fries.
Here you can see my DH getting ready to tuck in.
Once you've eaten, there are a very few seats with foot rests at the airport, if you want to put your feet up for a bit and are lucky enough to snag a couple. To get to them, head towards the electronic passport gates but then turn left as you draw level with them. Go between the newsstand on your left and a coffee shop on your right and you'll find the seats scattered around that area. There are toilets there too and that whole area tends to be much quieter than the rest of the departure lounge.
Note that there can sometimes be a long line to get through passport control and there is also document screening at the gate prior to boarding your flght back to the US, so give yourselves plenty of time to do that.