For the first verse only two parts are played to illustrate the canon devise. For the second verse the full four parts are played in full - Note that the first 4 notes of the Tenor part are the same as the last 4 notes of the Soprano part, and that the Bass part also echoes parts of the theme. The third verse introduces an organ accompaniment.
If you can't see music above, then install the easy to use Scorch Plug-in from Sibelius software.