If the margin requirement to trade 1 ES contract is $4,000 then with $4,000 in an account the most you can trade is 1 contract ($50/pt).
With $4,000 in an account and no fixed margin requirement, other than the amount of cash available, you could trade a lot more contracts. Say you trade 10 contracts you only have 8 pts of freedom before the entire deposit runs out and your position is closed for you crystallising a loss which wipes out the account.
Of course, if you have a proper trading plan then the relaxed margin requirements can work for you, but the company is aimed at traders who are under-capitalised and want to trade larger size. Which ultimately will lead to the account being quickly emptied.