Did I ruin my transmission driving in D3 and what is the difference between D3 and D4?

I was driving home today from work, a 24 mile drive, and was noticing that my RPM gauge was running high at 4500rpm while doing 70mph down the highway. At first I thought it might be my transmission going bad, then about 3 miles from home I realized I was in D3 instead of my usual D4. I didn't want to switch over while driving down the road since I thought it would drop the transmission like all the stories I have heard.

What is the difference between 2, D3, and D4? I just know to put it in D4 and drive. Did I ruin my transmission by driving in D3? The car is a 1997 honda civic with 98,500 miles.

Read more: Did I ruin my transmission driving in D3 and what is the difference between D3 and D4?