Hey there, hope this helps a bit.
Divide the interval from (0,4) into 4 different sub intervals. Notice that you will get the following smaller intervals. (0,1), (1,2), (2,3), (3,4) and each interval has a left endpoint and a right endpoint. Notice the width of each interval is 1.
a) find what f(x) is at the right endpoint of each sub interval. You now have the dimensions of each rectangle. Width is 1 and length is f(x) at each right endpoint. find the areas of each rectangle and add them up to find approximate area under the curve.
b) Do the same as part a but with left endpoints.
c) find the midpoint of each subinterval and find f(x) at each midpoint. then multiply the width of each interval times f(x) at each midpoint and add up those areas. You have approximated the area under the curve using midpoint method.
If you have questions send me a message and I'll provide further explanation.
Best of luck!1
