Box 4: Value on Exercise Date: This is the fair market value of the company’s stock on the date of exercise, as determined by the company’s stock plan documents.
Box 5: Number of Shares: This is the number of shares exercised (purchased) on the exercise date. This number may or may not equal the number of shares sold, if any, on that date.
Box 6: IF APPLICABLE (only applies if an individual is employed by a subsidiary of a company). The information for this box appears in the upper section of the form; this box captures the name, address and tax identification number of the parent company, the stock of which was purchased in this ISO exercise. If an individual is employed by the issuer (parent) company, then this box is left blank.
The 3921 Form deadline for issuing forms to participants is January 31st. The deadline to file 3921 Forms with the IRS is February 28th if filing on paper, or March 31st if filing electronically. If the deadline date falls on a Saturday, Sunday or legal holiday, the deadline would be the next business day.
For more information, Form 3921 instructions can be found at https://www.irs.gov/pub/irs-pdf/i3921.pdf