DD is 26th August born & tbh I never had any qualms about her going to school as soon as she turned 4. She (and I!) were sooooo ready!
From approx 6 months old, we did a lot of playgroups and socialising and even enjoyed communicating through 'baby sign'. At 2 years, she went to nursery also for 2 afternoons per week and started to have her days more structured. From age 3, she moved to pre-school for 3 full days, which built on her confidence, introduced a firmer routine and consolidated the reading/writing teachings we were doing at home. Therefore, by the time she turned 4 she was bored and needed a new challenge.
At first, the full 5 day week tired her out - in fact, she struggled with being over tired, but it was nothing that a good bedtime routine couldn't sort out, once established. However, she never had any problems with what was thrown at her in school, she held her own at reading, writing & numeracy.
Having said all that, both my DH and my brother share birthdays at the end of August too and they both state that they did feel like they struggled all the way through school. Despite this, they both are well qualified in their chosen lines of work and it has certainly not held them back in life. I suppose knowing this first hand before DD was born ensured that I took a very active role in supporting her learning & confidence building from a very young age to ensure she was fully prepared for schooling when the time came.
I now worry about LO #2, also due the end of August, but not because they might have to go to school at just 4, but what if they are born late & miss the deadline and then need to start school a whole year later at almost 5? I honestly could not imagine how bored the child would be!!!.