It's a bit of a cruddy question because two of the options are somewhat correct.
it definitely does not mean to be extremely busy, as someone else has suggested.
It is a combination of impatience and speaking too soon.
Richard is eager to see his brothers dead so that he can take the throne. But his planning about what he will do when they are dead is getting ahead of himself -- as he say, there is no point counting his gains until they are dead. In doing so, he is running before his horse to market.
I would say, therefore, the most accurate fit is to speak to soon, but being very impatient sort of fits also.
Definitely not 'to be extremely busy' or 'to make devious plans' or 'to be in denial'