Decoders usually (always?) come with CV29 set for the decoder to use a two digit address, specifically 03. A decoder stores a two digit address in CV1, and stores a four digit address in CVs 17 and 18. So a decoder can be programmed to have both a 2 digit and a 4 digit address, and the setting of CV 29 determines whether the 2 digit or the 4 digit address will be used. To switch to the 4 digit address, add 32 to whatever is in CV29. To switch from a 4 digit to a 2 digit address, subtract 32 from whatever is in CV29.
Hope this helps