There are no chords called C1, C2, C3, or C4. In the tab the author just means different versions (voicings) of a C chord; he could as well have called them C_A, C_B, etc. One of those voicings is actually a Cadd9 chord (from low to high):
x 3 2 0 3 3
(the note on the third fret of the b string is a D, i.e. the 9th of C). Note that this chord is different from a C9 chord, the latter being a dominant ninth chord with a minor 7th (a Bb).
There is also no C4 chord, only a Csus4 (suspended fourth), where the third (the E) is replaced by the 4th (the F).
Ed Sheeran actually uses only two types of C chords in that song. The 'standard' open string C chord:
x 3 2 0 1 0
and the Cadd9 chord shown above.