One easy solution if the choices for your variable are relatively stable is as follows:
- Create a parameter called say, Chosen_Var, containing the values that you want the user to choose between (i.e. A, B, C, D). Parameters can hold a single value.
- Create a calculated field called say, Var_Desired, to distinguish whether an individual data row meets your filter criteria
[Var] = "E" or [Var] = [Chosen_Var]
- Place that field, [Var_Desired], on the filter shelf and select only the True value
- Show your parameter control and configure as desired
This will allow users to select one of the values A, B, C or D, and then filter to only include data where [Var] = the fixed value E or the value the user selected.
If the set of legal values changes frequently, so that using a static list of parameter values is difficult, or if you want the user to be able to select multiple values, you'll need another solution.