If your work will be 100% performed within New York State, then file NY Form IT-2104 with your employer. Form 2104.1 is for non-resident employees who will be performing some of their work outside the State of New York.
File a NJ W4 if you want your employer to withhold both NY and NJ taxes. If your job will be your only source of income, then it may not be necessary for you to have NJ taxes withheld. That's because you will be able to claim a credit on your NJ tax return for the taxes you pay to NY on the income taxed by both states. The credit may be sufficient to cover your taxes due to NJ, since NY's tax rates run a bit higher than those of NJ.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
