Why has my friend been locked up for more than four months without any news? Why hasn't he been sentenced?

This is a criminal case. It usually takes about half a year to close the case. After sentencing, the time spent in the detention center is getting lower and lower. It's no use spending more money after sentencing. It is estimated that the case will go to the procuratorate in four months. If the situation you described doesn't look serious, it would be nice if the procuratorate didn't prosecute them. You'd better find a lawyer to meet your brother.