You can amend your 2017 to take the loss. In that case no 4684 for that should be in your 2018 return.
4684 Section D is not supported in Step-by-Step. You can open it in Forms Mode - click the Open Form button above the forms list on the left and scroll down to the 4684.
2017 TT will handle it. Select the This event does not qualify as a disaster casualty. That means it's not a "qualified" disaster with special treatment like the 2017 hurricanes/wildfires get. Your loss will be subject to the 10% of AGI haircut on Form 4684 line 17. Any excess loss on line 18 will be an itemized deduction on Schedule A line 20.
As you mention, prepare Form 4684 pg 4 Section D in the 2017 return (Desktop TT for Forms Mode, not Online).