Aulas de inglês para crianças de 4 anos: Por onde começar?

Perhaps a good start would be to get an English Picture Dictionary, there are some aimed to children audience.
It´s a way to speak to and with the kid, you could point a picture and work some conversation skills like saying "this is a house/and this is...is a car/that is a ..." and so on. There are material, like books with CDs with little songs, etc.

Speaking of which, you could use some nursery rhymes, or some songs like those. It´s good, since you don´t need think of it as something that the child has to answer to (as a test/exam, etc). The kid will learn almost by osmosis, and it´s a time to see their pronouncing skills. Making corrections only when necessary.
Most of the time, correction is made only by singing it along in the right way, and with time (and repetition) their will pick the words.
Now I leave the ball on the real experts, the ones that are teachers and education professionals.