When there are 4 or more monsters on the field: Destroy all face-up monsters.
Rulings
Individual Card FAQs
If the number of monsters on the field is reduced below 4 after activation of "Needle Ceiling" (by chaining "Ring of Destruction", etc.), the effect of "Needle Ceiling" still resolves (see "Activation & Targeting Eligibility" on the Advanced FAQ).
Face-down monsters and Monster Tokens are included in determining the number of monsters on the field.
Trap|
Normal