You have two options:
@discobot roll 4d6
which should provoke a separate message in reply, and
{roll 4d6}
using square brackets rather than braces, which will change the ownership of your post to “System” once the result has been inserted. See Dice roller ahoy! for more on that.
(If you want something that neither of these covers, asking me to extend the second one is more likely to work than getting the Discourse developers to extend the first one.)