Does the accountant bear a sin if he works for a company that deals with Riba-based loans, despite his advice to them, knowing that he owns 4% of its shares? And is he obligated to leave it, even if he loses half the value of his shares?

A loan with interest is usury, forbidden by the consensus of Muslims. If your percentage of participation in the company allows you to refuse borrowing with usury, then it is obligatory for you to refuse. You would be sinful by approving it, especially if you were to assist in it in your capacity as an accountant, due to the Prophet's (peace be upon him) saying: "Allah curses the one who consumes usury, the one who feeds it, its scribe, and its two witnesses," and he said, "They are all equal."

However, if you cannot object to it and do not assist in it in your work, then there is no sin upon you after offering advice, for Allah Almighty says: "O you who have believed, upon you is [the responsibility for] yourselves. Those who have gone astray will not harm you when you have been guided."

As for leaving the company: If the usurious borrowing is not part of the company's fundamental policy, then it is not necessary to leave. Otherwise, it would be obligatory, unless your departure would result in a severe loss, in which case delaying is permissible out of necessity. However, it is not permissible for you to calculate these interests in your capacity as an accountant.