Given the length of your race, you need to be doing intervals longer than 4-5 minutes. You're race will essentially be an hour or more at threshold. If you really want to use the 4-5 minute hill, I'd ride 5-15 minutes at threshold to the foot of the hill, then ride the hill for the final 4-5 minutes at or above threshold.
If you do it for a total of 20 minutes, then 2-3 times. If you do it for a total of 10 minutes, then 4-6 times.
Of course repeats of a 10-20 minute climb would be better.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.