Well, I said so much about Iceland, I decided to leave the rest for another post.
Getting from Barcelona (or Madrid) to Lisbon: the only practical way is to fly. Train takes too long for this one, and costs more! From Barcelona to Lisbon, flights on Vueling, Iberia, TAP, etc. run from $35 one-way to about $80 before you hit the high-priced ones. I just looked in May; Vueling and Iberia have $68 in the morning and $35 at 7:30 pm. Madrid-Lisbon, there's a $40 mid-day flight, but most other flights are much higher.
If you're planning to go to Porto as well as Lisbon, the trains between them are good, and fares between Porto and Barcelona or Madrid are comparable to the Lisbon prices. One trick to make the Porto trip cheaper: Take a Lisbon regional train to Coimbra (the university town) and change there for a Porto regional train the rest of the way. You'll save about 40%, lose little if any time, and end your trip in Porto's downtown station, instead of the suburban one used by the high-speed trains.
Honestly, if I had only 4 days for Barcelona and Madrid, I'd spend them all in one or the other (Barcelona would be my choice, but some would choose Madrid because of the Prado and Reina Sophia and Bornemisa-Thyssen museums). If you do decide to split, the way that will cost you the least of your time would be the high-speed AVE train, taken after a whole day visiting; on arrival, check in to your hotel and go to dinner (it's really true that many restaurants don't even open until 8 pm)