The best thing to do is use the calculator on the IRS web site or fill out all the worksheets and all the instructions on all 4 pages of the W-4.
If you are planning on using the standard deduction and have no other unusual tax circumstances, you would enter $4,000 on step 3 and leave it as-is. The W-4 by default assumes you are the only working spouse, and the tax tables will account for the larger standard deduction for MFJ. If you plan to take itemized deductions, or have other unusual circumstances, you can follow all the instructions and worksheets to see if there is any further adjustment required.
