Definitely FUEL problem.
It wouldn't really be considered a 'bog' if it runs strong until it gets to 4,800 RPMs. That's a fuel starvation issue.
Check the fuel filter, pressure, pump, float level, etc.
Could even be a clogged screen in the tank or a leaking rubber hose between the tank and the metal line at the back.
The fact that it takes a few seconds to clear means it's NOT valves. They would clear immediately when the RPMs went down.
Good luck!
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