Answer is 2:1
Reason:
There are 4 outcomes we don't want (rolling 1,2,3 or 4) and 2 outcomes we do want (rolling 5 or 6)
This gives the odds against ratio of 4:2 or 2:1
This can be written out as "2 to 1" or "two to one".
Having 2 to 1 odds means that we're twice as likely to get something we don't want vs something we do want.
Side note: the odds in favor will have us flip the ratio.