The Latvian team started briskly but were overpowered as the game wound on by their far more experienced opponents who featured superstar Cristiano Ronaldo among their number and who got the goal procession underway with a penalty in the 28th minute.
Ronaldo missed a second penalty, lifting the spirits of the visitors.
Then Latvian hopes of a memorable result kindled with Arturs Zjuzins levelling the scores in the 67th minute.
Unfortunately then the tiring Latvian defense buckled and within two minutes Portugal were ahead again through Carvalho in the 69th minute.
Ronaldo extended the lead in the 85th minute.
And deep into injury time Bruno Alves delivered the coup de grace with Portugal's fourth goal.
The result leaves Latvia in fifth place out of six in Group B of the World Cup qualifying pools for Europe, with only Andorra beneath them and the Faroe Islands one place above them.