At 4 years of age, children are still picking up pre-reading skills such as pronouncing simple words, developing vocabulary and writing simple letters.
Most children start to read around 6 years of age.
That said, every child develops at his own pace and in most cases, there is nothing to worry about.
http://www.m.webmd.com/a-to-z-guides/features/when-should-kids-learn-read-write-math