I won't advise sending your kid to school only in kindergarten because it might be too late, especially if the kid has always been with parents for the past 4 years and suddenly you put your kid in kindergarten, he might find it difficult to adjust compared to their peers.
Putting your kid early also allows the teacher (professionals) to teach and guide them as the grow older. This sets a strong foundation in terms of basic manners and etiquette, cultural understanding and social interaction.
For example, if you're Chinese, you will most probably teach your kid about CNY and mid autumn festival, if you're malay you will teach your kid about hari raya, Indian will teach about deepavali. So education is very limited at home, but at school, the teachers will tell stories and have some art & craft for every festival so all the students get exposure to basically everything. Even regarding manners in school, teachers will teach the children how to share, how to queue up, how to take turns. Are you regularly enforcing it at home?
When it comes to playing games, at school, sometimes win sometimes lose. How will your kid behave? At home, most parents probably make their kid the winner and say "good job" or "well done" all the time. So how will your kid react if he loses a game? Can your kid control their emotions, will throw tantrum? Or is your kid generous to let others first? It's hard to evaluate this kind of things at home and it's better to let your kid "explore" himself in a social environment. Children might be angels at home but they might behave otherwise at school. And vice versa.
Also, if your child has any developmental issues like fine motor, gross motor or speech delay, ADHD, autism, etc.. The teachers will know it better and can recommend your kid for occupational therapy or other relavant early intervention programmes. You don't send your kid to school early, you won't know how well he is developing.
