Suppose you want to run a program which requires $40\%$ of its total execution time to be executed sequentially. The remaining portion of the program can be divided into $6$ equal parts that can be executed concurrently. Assume you are given a multiprocessor with $4$ cores which can execute four processes simultaneously $($one process on each core$).$ What is the maximum achievable speedup by using all four cores over using one core. Explain your answer.