Max governor RPM (aka "high idle") should be something like 5000 rpm to the best of my memory. So, a little more than you are seeing now, but not a big difference. Maybe a small enough difference to be within the margin of error of your tachometer.
Is the engine CAPABLE of spinning faster than 4800, 5000, 5200 rpm....? Probably yes, if everything is in perfect condition and adjustment (timing belts, valve clearances, etc etc). But there is not really any point other than making life harder for your engine IMHO. The torque curve is already falling off very steeply by the time you are turning that speed, as intake and exhaust restriction are on a steep upslope. So even though you are correct that peak HP is rated at 4800 RPM, in reality you will want to be changing gears right then because there is nothing to be gained from pushing beyond the peak. Most of us change gears well before then anyway since the flat torque curve of the diesel engine means that better "seat of the pants" real world acceleration often comes from staying closer to peak torque (~2400 rpm IIRC) in a higher gear than screaming near at the HP peak in a low gear.
If you were talking about a D24TIC engine with MANY supporting upgrades to assist better breathing (like improved exhaust, intake, turbo, intercooling, etc, maybe cylinder head porting) then you might be able to carefully take advantage of higher engine speeds. But not on an airflow-limited NA engine. And also, just because someone did something in a video on YouTube doesn't mean it was a good idea.....
If you are looking for more power, your time would be better spent swapping in a turbo engine or adapting turbo parts to your NA motor.
