The W-4 worksheet is 4 pages long and can be confusing especially the part where both spouses have income ... try these tools instead : 

http://www.irs.gov/Individuals/IRS-Withholding-Calculator  

 https://turbotax.intuit.com/tax-tools  

http://turbotax.intuit.com/tax-tools/calculators/taxcaster  

You should have been provided instructions with that W-4. They are clear and understandable. If you wern't provided instructions then you can get them at https://www.irs.gov/pub/irs-pdf/fw4.pdf