I thank you all who have commented, your input has really put me on the right track because I am trying to save fuel and to extend the longevity of the car's components as far as possible. It sounds like I was making some mistakes, thank God it is only 15,300 miles on my car so far so I hope I have not caused too much damage with my misguided downshifts into 2nd gear to invoke engine braking. I have done that maybe 40 times total.
One last question. In the same scenario of travelling at 40 mph, If I see light changing to red up ahead is it wise to downshift to 3rd to slow the car down a bit? Or should I just stay in 4th and press the brakes?