Nowadays, Greenwich Mean Time, abbreviated as GMT, is a time zone designation rather than a time standard. Time difference between time zones can be expressed by the GMT or UTC offset. In the UTC standard, there is a commitment to keep within 0.9 seconds of GMT, so that every few years a leap second is applied to UTC. In the age of the internet, these small clock calibrations are conveniently distributed via NTP.
UTC/GMT-4 is 4 hours behind Greenwich Mean Time (GMT).
The GMT/UTC minus 4 hours offset is used by quite a few countries in the Caribbean, with no change throughout the year, as no Daylight Saving Time is applied. The Eastern Time Zone of USA and Canada only have the -4 offset when Daylight Saving Time is in use.
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The following countries or regions use the offset -4 either in Standard time or Daylight Saving Time as indicated.