What is the amount that the brother must pay: is it 4000 euros or 16,500 Saudi riyals, especially with the change in the currency exchange rate?

If you lend your brother an amount in "Euros," then he owes you this amount in the same currency. The fluctuation of the Euro's value against the Saudi Riyal does not matter, as the debt is repaid in kind.

The Islamic Fiqh Academy has ruled that "The consideration in repaying debts fixed in a certain currency is by its like, not by its value."

If you both agree to repay the debt in a currency other than the "Euro," then it should be paid at the exchange rate on the day of repayment. This is based on the Hadith of Abdullah ibn Umar, may Allah be pleased with them both, and Sheikh al-Islam Ibn Taymiyyah's affirmation of the permissibility of compensation at the day's price.