It's normal. My son is turning four next month and still can't read sentences too. He is only familiar with words that are associated with the colours, shapes, things, etc. that he always encounter in school. (e.g. Red, Yellow, Blue, Cat, Dog, Bird, etc.)
My daughter is nearly 2.5 years old and also can’t read. So it’s normal. I have a Friend whose kid is 3.5 and reads fluently. I aim to start teaching my Daughter how to read from 3.