Each could be correct, though the interpretations of each sentence are different. Sentences 2 and 3 express unusual things, and so seem a bit strange to my eye.
In 1, the speaker is making a casual analogy to distance: they have gone half of the distance that would be required to finish the job, meaning they have completed about 50% of it.
In 2, the speaker would be interpreted as expressing the degree of work they are intending to complete. In contrast to some argument (still would be interpreted here as something like "even with what you said in mind, our position isn't going to change), the workers will only be finishing the job halfway: whatever work they've done so far, they are only intending to complete 50% of it or so.
In 3, the use of to suggests that she is trying to reach the breakfasts but has not gotten to them yet.(perhaps she is pushing through a crowd that is obstructing her, or completing some work which must be done prior to getting the various breakfasts).
In 4, the lack of to changes the meaning to being halfway through the process of getting the various breakfasts, suggesting that she has eaten (or at least taken food from) the various breakfasts that are available.