The Lowest Common Denominator of two fractions is the LCM (Lowest Common Multiples) of the two denominators.
Multiples of 4: 4, 8, 12, 16, 20
Multiples of 3: 3, 6, 9, 12, 15, 18
the first multiple they share is the LCM
In this case that would be 12
Therefore your LCD for this problem would be 12