Your tax return must be filed correctly according to what actually happened during the year, including marital status, dependents, credits and deductions. Your w-4 is simply a way for you and your employer to estimate the tax you will owe so it can be withheld. Nothing you put in the W-4 is binding. The best way to fill out a W-4 is to follow all the instructions and worksheets for your situation (it's a 4 page document even though the employer only needs the first page, the other pages are worksheets and instructions). Or use this IRS calculator to get a recommendation. What you eventually claim depends on all your situations, including marital status, having more than one job or a spouse that works, dependents and their ages, and any other unusual tax situations.
https://www.irs.gov/individuals/tax-withholding-estimator
