UInt4


 * A UInt4 value is an unsigned integer of 4 bits.
 * It can only be 0, 1, 2, 3, 4, ..., 15.
 * Units of UInt4 have a pre-defined cardinality of 16, which cannot be overridden.
 * There is no undefined value indicator for UInt4.
 * UInt4 is a Sub Byte Element.
 * The GeoDms can store 8 UInt4 values in 4 bytes.
 * UInt4 can be the Domain of an Attribute, which implies that that Attribute has 16 values.