How can an investment fund that guarantees a monthly profit of around 4%, with the bank buying goods on behalf of the client and guaranteeing their purchase from him, be compliant with Islamic Sharia?

Guaranteeing profit in transactions renders them impermissible, moving them from permissibility to prohibition. For example, stipulating a guarantee in a Mudarabah contract invalidates it and turns it into a loan. Therefore, it is not permissible to participate in investment funds that stipulate a profit guarantee.