Ex7 Step 4 should end with an exercise summary

https://amazon-dynamodb-labs.com/design-patterns/ex7adjlists/step4.html

We shouldn't just move on to the next exercise without a summary of what was learned.

Open the Step 4 page linked in the issue and read the exercise flow around its ending. Add an exercise summary before the next exercise begins, and confirm the page clearly explains what was learned before directing readers onward.

Written by the indexing model from the issue text.