Category 4 is the airworthiness path in §107.140. The small unmanned aircraft must have an airworthiness certificate issued under Part 21, it must be operated within the approved Flight Manual limitations, and those limitations must not prohibit operations over human beings. The maintenance rules in §107.140(c) have to be met, and a consumer aircraft that is only registered under Part 107 is outside this category.
The rule
Section 107.140(a) tells the remote pilot to use an aircraft that is eligible under paragraph (b) and to operate it inside every operating limitation the Administrator specifies. Paragraph (b) is the certificate, the Flight Manual, and the maintenance performance. There is no Category 4 label and no Subpart D Declaration of Compliance. Section 107.160 is the applicant's declaration for Category 2 and Category 3. The 0.55-pound path, with no further paperwork of this kind, is Category 1. An airline transport pilot certificate does not put an aircraft into Category 4. Closed-site limits are a Category 3 operating condition, not the definition of this category.
Sustained flight over an open-air assembly is still conditioned. Section 107.140(a)(2) says no remote pilot may conduct that sustained flight unless the operation meets §89.110 or §89.115(a). The Flight Manual can be more restrictive than the regulation. If the manual prohibits flight over people, the aircraft is not eligible for this category even though a certificate exists.
Maintenance is part of eligibility, not a suggestion. The owner meets §107.140(c) unless the owner and an operator agree that the operator will. The person who performs the work uses the manufacturer's current maintenance manual or Instructions for Continued Airworthiness acceptable to the Administrator, or other methods acceptable to the Administrator, and has the knowledge, skill, and equipment to do it. Parts have to leave the aircraft at least equal to its original or properly altered condition. Records of the work include the description, the date, and the name of the person who performed it. The owner also keeps the status of life-limited parts, the inspection status, and the status of applicable airworthiness directives, including the method of compliance, the directive number, and the revision date. Recurring directives need the time and date of the next action. Work records are kept for one year after completion or until the work is repeated or superseded. The status records stay with the aircraft when ownership changes. The FAA and the NTSB can inspect them. Compliance with Part 43 and Part 91 subpart E fulfills paragraphs (b)(3) and (c). The small UAS maintenance page is where ordinary Part 107 recordkeeping, which is not this Category 4 duty, is separated out.
How to apply it
Start from the certificate and the manual, not from the job. If the manual's limitations allow operations over people, fly inside those limitations and inside the rest of Part 107. If the manual is silent or contrary, you do not invent Category 4 eligibility because the client needs the shot. Confirm the maintenance status before launch: inspections current, life-limited parts tracked, airworthiness directives recorded. Then apply the assembly sentence only if the path is sustained flight over an open-air assembly.
Since April 21, 2021, an operation that meets a Subpart D category does not need a §107.39 waiver. That includes a Category 4 flight that fully satisfies §107.140. It also includes a Category 2 flight that makes the same point: once the category's own requirements are met, the over-people waiver is not an extra approval. LAANC and a one-time letter from the Flight Standards District Office are airspace or other tools. They are not a substitute requirement for a compliant category flight, and they do not fill a missing airworthiness certificate.
A propeller and a certificate
A Category 2 aircraft needs a propeller replaced before a flight over people. The remote pilot operating instructions list that propeller replacement as an allowed modification. AC 107-2A paragraph 8.4.5 says maintenance that does not change the aircraft's configuration or characteristics does not require a new Declaration of Compliance, and that listed propeller is the example. You follow the instructions and keep flying Category 2. You do not send the aircraft for a Part 21 certificate to "make it Category 4," and you do not file a new declaration for a modification the instructions already allow. A propeller change those instructions do not cover does require a new Declaration of Compliance before further Category 2 or Category 3 operations over people. That new declaration still leaves the aircraft on the Category 2 or 3 path.
A different aircraft in the fleet has a Part 21 airworthiness certificate, a Flight Manual that permits operations over people, and a maintenance record that meets §107.140(c). That aircraft is the Category 4 aircraft. The pilot operates inside the manual and does not apply for a §107.39 waiver merely because people will be below. The four-category map sits on operations over people. The declaration path sits on Category 2.
The trap
The eligibility trap offers a Declaration of Compliance and a Category 4 sticker, or a weight at or below 0.55 pounds, as if either one were §107.140. Category 4 is the Part 21 certificate, Flight Manual limitations that do not prohibit flight over people, and the maintenance rules in that section. The sticker-and-declaration package is Category 2 or 3. The weight is Category 1.
The maintenance trap says every propeller change voids Category 2 until the FAA issues an airworthiness certificate, or that every allowed modification needs a fresh declaration. The allowed propeller stays inside the operating instructions and does not create Category 4. The waiver trap says every flight over people still needs a §107.39 waiver or an FSDO letter. Since April 21, 2021, a flight that meets the category is routine. Meet §107.140, or do not call the flight Category 4.
Use the free practice test to separate the certificate from the declaration. The course teaches Category 4 from the Flight Manual and the record rules, not from the weight of a hobby aircraft.