In reply to bachan21:
makes a difference as randomization is compute heavy
This is not correct. Randomization only works with 2-state values.
Unless you are working with very large objects, there is not much difference between 2-state and 4-state datatypes in terms of performance and capacity. You need 4-state values if you have to deal with Z-states, or need to check that X values have not propagated to the outputs. But for most stimulus generating classes, you only need 2-state data types.