Update the 4th meetup

[hwchiu]

Update the 4th meetup, including links of Youtube and slide.

Checklist

• [ ] Fork the repository and create your new branch from master.
• [ ] Please mention the name of community in Pull Request title.
• [ ] Git Commit Messages
  • [ ] Use the present tense (Also in Pull Request title): "Add feature" not "Added feature"
  • [ ] Use the imperative mood (Also in Pull Request title): "Move cursor to..." not "Moves cursor to..."
  • [ ] Use rebase to squash/fixup dummy/unnecessary commits into only one commit.