Any neat way to limit significant figures with BigDecimal

You can use the following lines:

int digitsRemain = 4;

BigDecimal bd = new BigDecimal("12.3456");
int power = bd.precision() - digitsRemain;
BigDecimal unit = bd.ulp().scaleByPowerOfTen(power);
BigDecimal result = bd.divideToIntegralValue(unit).multiply(unit);

Note: this solution always rounds down to the last digit.

The easierst solution is:

  int newScale = 4-bd.precision()+bd.scale();
  BigDecimal bd2 = bd1.setScale(newScale, RoundingMode.HALF_UP);

No String conversion is necessary, it is based purely on BigDecimal arithmetic and therefore as efficient as possible, you can choose the RoundingMode and it is small. If the output should be a String, simply append .toPlainString().

Why not just use round(MathContext)?

BigDecimal value = BigDecimal.valueOf(123456);
BigDecimal wantedValue = value.round(new MathContext(4, RoundingMode.HALF_UP));