There aren't many catches to a 401(k) plan. But the one big catch is that once your money is in the plan, you'll have to pay a penalty if you withdraw the money too soon (generally before age 59½), as well as the ordinary income tax you will pay on any withdrawal. See the section Your 401(k) When Switching Jobs for information on the early withdrawal penalty. To be safe, plan on leaving the money and its earnings in a qualified plan until you retire (no earlier than age 55) or reach age 59½.
Early Access to your 401(k) savings
If you decide that you really need to take money out of your 401(k) plan to which you contributed pre-tax dollars, there are three ways you can do so.
If you are over age 59½ or meet other exceptions to the 10% early withdrawal penalty, you can withdraw your 401(k) money without penalty provided your employer's plan allows such distributions. You will have to pay ordinary income tax on the amounts received. See the section Your 401(k) When Switching Jobs for more details.
Read the section Pre-Retirement Withdrawals for more details about loans and hardship withdrawals. Also see the section Distributions from your 401(k) at Retirement for details on taking retirement distributions.