A 504 Plan refers to Section 504 of the Rehabilitation Act. Section 504 is a civil rights law meant to protect children and individuals with disabilities from being discriminated against because of their disabilities. Children who have a physical or mental impairment which impacts their education may have the right to receive a 504 Plan. A 504 plan entitles these children to reasonable accommodations and/or modifications (i.e. extra time on tests, a piece of technology or medical equipment) to allow them to access their curriculum — to bring them to the same level as their same-age, non-disabled peers.
If your child requires specialized instruction (i.e. a certain methodology in order to enable them to learn) then your child needs an IEP – not a 504 Plan.