If principal applicant of approved petition of I-130-F4 dies, how can his spouse and two children(12,10) try to get immigrant visa from Bangladesh?

Under the circumstances that you describe, your brother's family will not be allowed to immigrate to the US on the basis of the I-130 for your deceased brother. That is because the relationship is between you and your brother, and the family members are only dependents. I do not know of any exception where the beneficiary's family is overseas. Perhaps his wife could be sponsored under an employment visa if she has skills that are needed in the States. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.