Fourteen year-old Ahmed Metwaly died on Saturday of wounds sustained during clashes between President Mohamed Morsi's supporters and opponents in Alexandria on Friday.
So far, three were killed, including a US citizen stabbed in the chest while taking pictures of the violence with his mobile phone.
The Brotherhood headquarters were torched amid clashes in front of their offices in Egypt's second-largest city.
Egypt has witnessed nationwide clashes between the president's supporters and opponents ahead of anti-Morsi protests scheduled on the year anniversary of taking seat, 30 June. Demonstrations are expected to be the largest protests since Egypt's 2011 revolution.
Islamists supporting the president have already gathered at the Rabaa Al-Adawiya Mosque in Nasr City, a densely-populated Cairo suburb, since Friday. This is only a few kilometres from the presidential palace where opposition marches are planning to converge on Sunday.