TIdDecoder4to3 = class(TIdDecoder);

TIdDecoder4to3 is a TIdDecoder descendant that implements decoding capabilities for data encoded using TIdEncoder3to4. TIdDecoder4to3 provides a decoding table to represent the valid characters in the encoder alphabet. 

TIdDecoder4to3 provides methods to decode string values to a destination stream, and methods to handle the conversion performed when processing string input values. 

TIdDecoder4to3 does not specify the valid characters in the decoding table. Application will generally use a descendant class, like TIdDeCoderMIME or TIdDecoderUUE, where the alphabet is specified.