Glossary
Q4 (overload flow rate)
The highest flow rate at which a meter can run for a short period without damage while staying within its maximum permissible error. Under ISO 4064 and OIML R 49, Q4 is 1.25 times Q3.
Why it matters
Peak demand, such as filling an overhead tank after supply resumes, can push a meter past Q3. Q4 tells you how much short-term headroom it has.
Source
ISO 4064-1:2014; OIML R 49-1:2013