The time Isn't the problem. The cost to do 4 semesters is basically 30000CAD. I'd rather do a 1 year program.You will not get offer for your second and third choice if you have been accepted for your first choice.
You have two options, either contact them and see if they will revoke this offer letter and go for your second or third choice.
Or you could apply again for the program of your choice. You will have to pay application fees again.
I am assuming you are talking about the diploma program, 4 semesters seems to be a decent amount of time. What is the issue you are facing ?
Also what are your second and third choices ?
What I actually wanted to do is Marketing Management not Business Marketing so I made an error. Second Choice is project management, third choice is Insurance property and casualty