One of 2 candidates will be elected, despite there being 4 candidates running. That is a fact.
You have several choices to make.
1) You can vote for the candidate you like most, despite having no chance chance getting elected.
2) You can choose not to vote.
3) You can choose one of the 2 'viable' candidates that either:
a) most closely resembles your ideals over the other 'viable' candidate or
b) minimizes the 'losses' you will feel over the other 'viable' candidate.
http://www.socsci.uci.edu/~bgrofman/R7-Grofman-Abstention%20in%20two-candidate%20and%20three-candidate%20elections.pdf