4 is deduced from 1,2,3 the deduction is just using definition and the fact both parties lead to the same outcome to come to a contradiction
Both parties have been elected (1), both led to (2), both aren't representative (3), You can't blame voters if they have no power over the outcome, there's no democracy in US
If the point you're trying to make is that outcome is irrelevant and only voting itself is what constitutes a democracy then this needs to be applied to other countries as well, North Korea is a democracy