Step 6 -- Produce a plot of the solution

Input := 

Y[x_] := Which[ 0 <= x < 30, y1[x],
               30 <= x < 60, y2[x],
               60 <= x < 100, y3[x],
              100 <= x <= 150, y4[x]]

Input := 

Plot[ Y[x], {x,0,150}]

Output =

-Graphics-