Taking some assumptions , let the 5 stages be IF, D, EX, M ,WB. Since in the case of pipeline it is given that branch will not be known till M, so there will be 3 stall cycles. Branch penalty=3(for branch instructions).
IF | ID | E | M | WB
X | X | X | IF |...... so, 3 stall cycles will be there in case of branch instruction.
Avg CPI(pipelined)= 1(in the ideal case) + Branch penalty *# of branch instructions=1+3*P (let P will be the number of branches
instructions)=1+3p.
In the case of non-pipelined CPI=5(since there will be no overlapping).
Given speedup>=4
=> 5/1+3p>=4
=>p<=0.084