Since you put 4% of your income into a 401k, your wages subject to tax are $48,000.
It is true that you are in the 25% tax bracket. This does not mean that you pay 25% of your wages, or $12,000 in federal tax. Nor does it mean that should you be able to reduce your tax bracket to 15%, that you would save 10% of $50,000, or $5,000 (or $4,800 if you figure your wages as $48,000).
With income of $48,000, your taxable income is $38,000 using the standard deduction of $6,100 and one exemption of $3,900. The 25% tax bracket starts at taxable income of $36,250. So you pay 25% tax on $1,750. The rest of your income is taxed at a maximum of 15%.
If you had itemized deductions of $7,850, you are now in the 15% tax bracket, and you have reduced your tax by 25% of $1,750, or $438.
If you had itemized deductions which were $2,800 over the standard deduction, or a total itemized deductions of $9,900, you would save $595 because only part of the $2,800 would have been taxed at 25%, the rest is taxed at 15%.
If you want to get out of the 25% tax bracket, see if you can put an additional $1,750 into your 401k. You would increase your 401k by that amount, and decrease your taxes by $438, so the additional contribution reduces your take-home pay by $1,312.