What is the ruling of Islamic law on taking 4% of the profits from the money of my relatives who have invested their money with me, in exchange for 4% of the monthly profits, which are originally 8%?

What you did by investing your money in the pharmaceutical trade is called Qirad or Mudarabah (commenda partnership). This contract is invalid due to the guarantee of the capital and the specification of a percentage of the total capital, and not of the profit.

If the Mudarabah is invalid, then the owner of the money receives the profits, and the worker receives ujrat al-mithl (a reasonable wage for comparable work). This is the view of the Hanafis, Shafi'is, and Hanbalis, and one of the two opinions of the Malikis. Others held that he is entitled to the profit of the like (ribh al-mithl), which is the other opinion of the Malikis and the choice of Shaykh al-Islam Ibn Taymiyyah.

You have committed two prohibitions in your dealings with your relatives: involving them in an invalid Mudarabah, and concealing the truth from them. It is obligatory for you to repent from this, and all the profits belong to your relatives, while the trader receives ujrat al-mithl.