How to create custom 404 page content
From version 3.6.0 you can create a custom block and assign it to the 404 page to replace its content and edit it with the builder.
Create a new block
Navigate to Blocks → Add New and create a new block (you can use any title). Add some content to your liking and save the block.
Assign the block as 404 page content
Navigate to Advanced → 404 Page and assign the new created block.