Improve explanations in section 4 (and in solutions)

[bencomp]

Based on questions from participants:

• use metaphores to explain why people would want loops
• solution to exercise 4.2 has incorrect grammar: "which print all each characters of a string on a separate line"
• solution to 4.2 does not explain why the solution works
• solution to 4.3 could use comment to explain += shorthand / show alternative solution
• solution to 4.4 could use comment to explain the logic that makes it work
• solution to 4.5 could use comments to explain the logic; this is a difficult exercise and people find it hard to come up with this