If your liability limits are exceeded can they go after your 401k?

I believe 401k is protected. However, any unprotected assets such as 2nd homes, brokerage accounts, etc. could be vulnerable. Assuming the injured party has retained counsel, an asset search was made. And if he/she/they have counsel, you probably do, too. Ask your lawyer, or get one. If the claim is against an auto or homeowner's policy, the insurance company provides an attorney at no cost to you. That attorney, however, really does work for you and will protect your interests.