Our organization administers a 401(k) plan with about 400 participants. A small division consiting of 4 employees will be sold to one of the employees. We would like to allow these 4 participants to become 100% vested in their 401(k) accounts upon the sale.

Any suggestions regarding where to look for the requirements to vest these participants?

Off hand, I do not believe that you would be required to 100% vest the four participants. You could, though, amend the Plan to 100% vest the four participants if you want.

Note also that as part of the sale, you could transfer the assets and liabilities of the four participants from your 401(k) plan to a 401(k) plan established by the new business. In that case, the four participants would continue to vest in accordance with the vesting schedule.

Right. If you want to give them vesting, the best way to do so is amend your plan.