Fill up the form I-539 and mail it to BCIS with Fee check, your Valid H1 approval copy and I-94Copy( front and back)
Spouse should also send the copy of her H1 approval and copy of I-94 ( front and back)
write in cover letter in bold: Form I-539 , Change of Status from H1b to H4
and write the copy of supporting documents that are attached with the Form.
Also write out side of the envelope in bold big letters: Form I-539, Change of Status from H1b to H4