I am a Canadian citizen and I am flying into Minneapolis from Toronto on a one way ticket. I am originally from Winnipeg (Manitoba, Canada) and plan to drive from Minneapolis to Winnipeg with a friend. I will only be in MSP (Minneapolis–Saint Paul International Airport) for four days.
Do I need to prove onward travel? If so, how do I do that?
No, as a Canadian citizen you do not need to prove onward travel when entering the US. The CBP officer will probably ask you what your plans are, but it's totally normal for Canadians to do the sort of thing you're doing.
Having said that, your individual experience will all come down to the CBP officer who interviews you at the US preclearance facility in Toronto. Unfortunately, if he or she does not like something about you or your plans, they have the unilateral authority to deny you entry. From what you've said about your plans though, denying you entry seems unlikely.
