Some children are very interested in reading at 3 or 4. If this is the case, there is nothing wrong with starting with them - the idea that only pushy parents would do this is daft - lots of children are capable and ready to learn some basics at 3 or 4 and some won't start school until they are very close to 5, so why not.
It is true that children who haven't done it before school, who turn out to be good readers will catch up with those who started before, so in the end it doesn't matter.....however, being a bit ahead or on top of it, if it hasn't been forced by parents, can be helpful for confidence building and just boosting early reading skills.
To give my DC as example...they started learning phonics at 2.5 - the basic stuff. They wanted to and had a collage thing on their wall and used to point at the phonic sounds and I'd tell them what they were and very very quickly, they just knew them all.....no instigation from me, apart from it was us who stuck it up for decoration. And at 3.5 were doing some basic blending and reading words, followed by basic books of both phonic and non phonic nature - some phonics schemes, but also Peter and Jane which isn't phonics and not liked by many. Both DC were Autumn born and by the time they started school were able to read basic story books and Enid Blyton .....could read pretty fluently out loud, with understanding.
At school, they started with a a a ant and the jolly phonics actions - all fine ....enjoyed the group activities, were quickly spotted as readers and moved onto about Level 6 of the reading scheme. Yes, they sat through lots of carpet time on basic phonics, but I and they weren't bothered by that. The teacher/TA heard them read the school scheme books (easier than they would read at home, but fine) and at home they continued to read more advanced stuff every day.
Years later.....they are good and avid readers. Some other kids their age, who couldn't read a word when they started school are too, and are equally good and avid. One benefit I think mine had was always having a sense they loved reading, always feeling they were good at it and doing well and never really going through the struggling to become fluent phase or not wanting to read nightly. By the time they started school, they read well enough and regularly enough for it not to be a chore and to be normal.
In the end it will be a personal choice. Whenever they start to read, there can be a hard phase where they move to fluency. Whenever they start, I think the key thing is for parents to get the children to read every day, pretty much 365 days a year - it's not homework or a chore or something to miss at weekends or holidays, because it's a bit hard and they don't like it...it's like brushing your teeth and you do it every day. In my mind, parents who do that and who persevere through the early harder times, find their kids make quicker progress and are more likely to become confident readers who love it.
I'm always surprised at how few parents these days do any reading with their pre-school children. Knowing where to start can be off-putting, as can a fear of 'doing it wrong'. I think once children start school and phonics and move into books, lots of parents find it hard work...children resist or struggle to move to fluency...it's a bit of a battle and so parents opt out at points - especially school holidays....and this makes it hard to sustain progress and makes reading seem like a chore and homework to children....so my advice, is that whenever you do start (might well be at school) then commit to the 365 days a year on it...it's so worthwhile.