There might be other ways though, like first encode 2 bits as one symbol and then two symbols as one 4-PSK symbol. then in theory you will be able to send 4 bits
maybe i have not ubderstand the question.
Its a problem which says:
a source produce 8 symbols with the same probability , 5*10^4 symbols per second.
If the outputs efficiency is 0.75( from this i understand that i need 4 bits pes symbol) and bandwidth=50KHz, how would be a telecommunication system with 4-psk which would permit the transmission through channel with frequency response |C(f)=a|.