I am married with a dependent but claim single/zero on my W4 in order to have enough withheld. My wife claims married/1.
What would be the impact to my Federal taxes if I switch to married/1 on myW4 and contributed the additional take-home pay to my 401K,?
If you change to add married 1 versus single and 1 to your W-4 at work your taxes withheld would go down on your paycheck.
If you change to add more to a 401K (if a Traditional 401K only - not a Roth 401K) your taxable income on the pay check would go down and your income tax withheld from your employer would also go down.
So if you did both of these (and nothing else changed), you would get a smaller refund (or owe taxes).