How to be more specific on generic constraints? [duplicate]

This question already has an answer here:

• Is there a constraint that restricts my generic method to numeric types? 20 answers

I am having the following type where T should only be of int, double, string and datetime. However using a constraint on class would be too broad.

public class Field<T> where T : class 
{
    public int Tag { get; set; }

    public T Value { get; set; }
}

Possible solutions that come to my mind:

1. Add a constructor that does a check (if ((typeof(T) != typeof(string)) ...)
2. Change the class to be abstract and move Value to the concrete implementing class. This however would result in having several additional types, like StringField : Field, DateTimeField : Field, etc.

Also I would like to be more specific about the Tag property and replace it with an enum, which ensures that only values of predefined range can be used. This approach however would result in a (tiny) overhead, as I need to instance representation of Field as a string.

What would be the best model to choose? Anything I am missing? Thanks in advance