Jamieson, Fausset and Brown on 1 John 1:4

4. these things--and none other, namely, this whole Epistle.

write we unto you--Some oldest manuscripts omit "unto you," and emphasize "we." Thus the antithesis is between "we" (apostles and eye-witnesses) and "your." We write thus that your joy may be full. Other oldest manuscripts and versions read "OUR joy," namely, that our joy may be filled full by bringing you also into fellowship with the Father and Son. (Compare Joh 4:36, end; Php 2:2, "Fulfil ye my joy," Php 2:16; 4:1; 2Jo 8). It is possible that "your" may be a correction of transcribers to make this verse harmonize with Joh 15:11; 16:24; however, as John often repeats favorite phrases, he may do so here, so "your" may be from himself. So 2Jo 12, "your" in oldest manuscripts. The authority of manuscripts and versions on both sides here is almost evenly balanced. Christ Himself is the source, object, and center of His people's joy (compare 1Jo 1:3, end); it is in fellowship with Him that we have joy, the fruit of faith.