A cotton cultivar, designated 4646B2R, is disclosed. The invention relates to the seeds of cotton cultivar 4646B2R, to the plants of cotton 4646B2R and to methods for producing a cotton plant produced by crossing the cultivar 4646B2R with itself or another cotton variety. The invention further relates to hybrid cotton seeds and plants produced by crossing the cultivar 4646B2R with another cotton cultivar.