Is there any further actions I need to take other than clicking Yes on the T4 form for the $400 working from home deduction?

You would also need to go to the Employment Expenses to answer some questions regarding eligibility and how many days you worked from home. This FAQ tells you how to claim Employment Expenses in TurboTax Online: https://turbotax.community.intuit.ca/community/claims-deductions-donations/help/where-do-i-claim-my-employment-expenses-in-the-online-edition-of-turbotax/00/873421

FYI: The $400 is the maximum amount of the deduction that you can get, but you may get less depending on how many days you worked from home. It’s $2 per day, up to $400 if you worked 200 or more days from home.