A novel soybean variety, designated XB004B12 is provided. Also provided are the seeds of soybean variety XB004B12, cells from soybean variety XB004B12, plants of soybean XB004B12, and plant parts of soybean variety XB004B12. Methods provided include producing a soybean plant by crossing soybean variety XB004B12 with another soybean plant, methods for introgressing a transgenic trait, a mutant trait, and/or a native trait into soybean variety XB004B12, methods for producing other soybean varieties or plant parts derived from soybean variety XB004B12, and methods of characterizing soybean variety XB004B12. Soybean seed, cells, plants, germplasm, breeding lines, varieties, and plant parts produced by these methods and/or derived from soybean variety XB004B12 are further provided.