This should do it:
int untilNextQuarter = 4 - (currentMonth % 3);
Or a slightly clearer but slightly less efficient approach:
int[] remainingMonths = new[] { 3, 2, 4 };
int untilNextQuarter = remainingMonths[(currentMonth - 1) % 3];
This should do it:
int untilNextQuarter = 4 - (currentMonth % 3);
Or a slightly clearer but slightly less efficient approach:
int[] remainingMonths = new[] { 3, 2, 4 };
int untilNextQuarter = remainingMonths[(currentMonth - 1) % 3];