N.L.
Maybe he really IS mad at Grandma... for not taking him back to Portugal! =-) At 4 years of age he doesn't understand the concept of travelling and what it entails. He thinks you simply drive to the airport, get on a plane and voila! They are in Portugal again.
You said he had a great time so why wouldn't he want to go back? We took our kids on vacation this past Summer and for the few months after my 4 y/o DS could not understand why we couldn't just get back on the plane and go there again. He was mad about it and we had to just keep explaining why. He's over it now.
Anyway just a thought. Forgive me if I'm way off base LOL!
BTW, have you asked you DS why he is so mad? At this age they can articulate quite a bit. Ask him NOT while he is angry but during cuddle time with you.