It depends on the child, if they are ready and keen you could teach them to read before they start school.
I did it with both my sons. DS1 was ready a few months after he turned 4. I wanted to teach him to read before he started school so he wouldn't be confused learning to read in two languages at the same time. When he started school he could already read in Spanish. He learned his phonics at school and progressed really quickly as many of the skills he knew from reading in one language were transferred into reading a second.
DS2 was ready a few months before he turned 4. I taught him to read in Spanish and only took him a couple of months to learn. We only spent 5 minutes a day playing phonic games which he enjoyed.
Once he knew he could read in Spanish, he kept trying to read signs, labels, posters, etc but I kept having to explain that some letters have different sounds if the words are written in English. As he was so keen to learn I got him some phonic flashcards and decodable books, and I only had to explain which sounds were different and teach him the ones that were exclusive to English. Again, he learned in just a couple of months and can now read books that DS1 was reading at the end of reception.
I agree with irvineoneohone, that reading is not a limited skill and you can increase vocabulary and learn reasoning skills while reading. Although DS2 can read a book in English, he doesn't always understand the words he is reading because they are not in his vocabulary, given that English is his second language at the moment. So our reading time is the perfect opportunity to talk about words he doesn't know, what they mean, and in turn develop other skills like understanding the story, predicting and talking about what characters are feeling, etc.