(As clearly mentioned using 4:1 , that means we have not to use any other MUX of some other type...)
Ans :- Two 4:1 MUX wil be used at first level , each of which can take 4 input (i.e 8 input are covered) and give 1 output each. so now we have to cover two input. since we have no other type of MUX . so we will use one 4:1 MUX (which will produce final output.)
Hence total 3 MUX will be required.
NOTE:- MUX at last level (i.e which will be producing the output) will not be used at its full capacity...