I've had it pop out on different bikes over the years. Most bikes have a false neutral between gears. While it may be difficult to find, it's not impossible. I'm going out on a limb here, but I suspect you may not have gotten 4th completely locked in. It may have cruised around for awhile and when you gave it some gas, or came off the throttle, it popped into the false neutral. It will for sure catch you off guard. I'd plan to ride it just like always. Then if it begins to happen regularly, it may be time to replace a gear that got chipped.
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