You can now do:
{{#if dj4name}} {{dj4name}} {{#if dj4twitch}}- {{dj4twitch}}@twitch{{/if dj4twitch}}{{/if dj4name}}
And, if dj4name is defined, it'll try to render
{{dj4name}} {{#if dj4twitch}}- {{dj4twitch}}@twitch{{/if dj4twitch}}
And, then if dj4twitch is defined it will render:
{{dj4name}} - {{dj4twitch}}@twitch
but of course with {{dj4name}} and {{dj4twitch}} substituted with whatever they are defined to. Of course, they don't need to be nested, I just wanted to show a more complex example.
There is also a better fix for the variable name labels on the variable definition dialog.