First split the interval ##[a,b]## into 4 equal subintervals:

##[x_0,x_1],[x_1,x_2],[x_2,x_3]##, and ##[x_3,x_4]##.

(Note: ##x_0=a## and ##x_4=b##)

The definite integral

##int_a^b f(x)dx##

can be approximated by

##T_4=[f(x_0)+2f(x_1)+2f(x_2)+2f(x_3)+f(x_4)]cdot{Delta x}/{2}##,

where ##Delta x={b-a}/4##.

I hope that this is helpful.