Do I need a work permit to start a business with my Thai wife, and will this affect my ability to import personal goods duty-free?

You need a work permit. It wont change any import duty that's payable

I believe you also need 4 employees in the business

Go and speak to the Labour Department. They can be quite helpful when they want to be.

Depending on the type of business you MAY not need one (incoming abuse alert).

All depends on the circumstances.

If you want to work in the business you need a work permit. If you won't work in the business you won't

No to the first one if you are actually not working. To import personal items it has to be done within 6 months of your first arrival for retirement, work, or any long term visa. If you were here on a tourist visa for long extended periods they could query your eligibility for tax free import. Tax exemption is only for household goods except for electronic goods, alcohol, plus some other items where you have to pay import duty. Nor anything you wish to resell is tax exempt.