The first part of your sentence is clear, the 'four to one' part really means nothing to me in this sentence.
There are two contexts I can think of where I'd expect to hear 'xx to one'. One is reporting scores of competitive sports, as in:
A: How did your baseball game go?
B: They beat us four to one/They won four to one. (or any number to any number)
The other is in the context of how many times more (usually) people there are compared to a different group of people, and is mostly used in conjunction with the verb 'to outnumber', (and most likely 'any number to one') as in:
"In our class, girls outnumber boys almost three to one." (There are almost three times more girls than boys)
"They outnumbered us two to one." (Probably talking about a group of people in a fight or battle - there were twice as many people on their side as on our side)
Thank you very much for your explanation. I do not know anything about horse gambling so I thought it is language issue. Now I know the information of horse gambling is important for you to understand the sentence.