First, 4ms is 3.4 orders of magnitude off 10 seconds.
Second, do you have any argument that isn't based on personal ignorance?
The stock market isn't even open at night, no one thinks that it is necessary to trade 20 times a second every second, they just want to. The stock market doesn't directly represent economic conditions, it represents investors assessment of economic conditions, thus it can change as fast as they can change their mind. If someone wants to only trade every ten seconds they can go ahead. If someone wants to open a stock exchange where everyone can only trade every ten seconds they can go ahead.
How does a machine taking advantage of small arbitrage opportunities that last fractions of a second represent an "investors assessment of economic conditions"?
Second, do you have any argument that isn't based on personal ignorance?
The stock market isn't even open at night, no one thinks that it is necessary to trade 20 times a second every second, they just want to. The stock market doesn't directly represent economic conditions, it represents investors assessment of economic conditions, thus it can change as fast as they can change their mind. If someone wants to only trade every ten seconds they can go ahead. If someone wants to open a stock exchange where everyone can only trade every ten seconds they can go ahead.