Career Planning Advice
You don't have to go into 4th year with only one specialty in mind: it's ok to have interests in a number of fields and to put forth CaRMS applications to all of them!
Even if you aren't sure about your candidacy for a given field, the simple fact that you are interested is reason enough to gather your nerve and apply to a residency program that specialty: by doing this, you give yourself the chance of an interview, a closer look at the field, a chance to meet some of the people you might be working with, and thereby you "buy some time" to mull things over for a few months, even deciding as late as February of 4th year.
Notice that this isn't really a trick or a tip to help you choose, but if you are feeling rushed and/or pressured, it's a practice that can give you the breathing room you may need in order to approach your decision more calmly and carefully.