Is the amount of 400 dinars that the questioner and his brother take from the money allocated to their two disabled brothers permissible, given that the two brothers support themselves?

The amount spent by the state on your two brothers is a gift to them, owned by whomever it was gifted to. Your brother does not have the right to take a part of it in exchange for withdrawing it, unless there is no volunteer to withdraw it for free; in that case, he deserves payment for his work. What your brother takes from this money and gives to you is not permissible for either of you, except with the consent of the two disabled brothers if they are adult, sane, and رشيدين (of sound judgment). The obligation is to repent and return the money to its rightful owners.