The four were arrested in a government crackdown after the group staged a wave of pro-reform protests across the country on 6 May. They were jailed for belonging to a banned organisation and for organising unauthorised protests.
The order extending the sentences of prominent Brotherhood figure Essam el-Erian and the three others was issued on Tuesday by Prosecutor Hisham Badawai.
El-Erian, a 52-year-old former lawmaker, spent five years jailed during the 1990s on charges of belonging to a banned group that sought to create an Islamic state in Egypt.
On Sunday, Mahmoud Ezzat, another high-ranking member of the Brotherhood, was released on bail after about three months in detention.
About 2000 Egyptians took part
The Muslim Brotherhood, banned since 1954, had long refrained from provoking the government but resumed protests in March, demanding political reform.
The Brotherhood, established in 1928, renounced violence in the 1970s.
Supporters of the group sit as independents in parliament, holding 15 seats as the largest opposition bloc.