Your estimated gross income $4500*12=$54000
Assuming you do not itemize and do not have any other deductions or credits - your standard deduction as single is $5450, personal exemption is $3500,
your taxable income will be $45050
and tax liability $7606
All estimation is made based on 2008 tax rates - in 2009 the tax liability will be a little less.
Let me know if you need any help.