What is supplemental tax rate? Laid off 12/31/16. Severance paid Jan ’17. Package taxed at supplemental rate. Do I change W4 prior so less taxes are taken?

Luckily severance comes after end of year.  I am trying to find out what the supplemental tax rate is (for 2017, or for 2016 as a guide) and whether I can lower the amount taken out of the severance pay by changing my W4 before the end of the year. I do not want the government to take out half or more of my severance.