I don't think the final diagnosis is going to be a happy ending type.
The video doesn't show any oil smoke coming from the exhaust.
Is this really the case ? Or did the camera angle just not pick it up ?
Have you tried removing the air filter and blocking the hose end with a thumb or finger in order to get some kind of feel for how much pressure is building up and how fast ?
Let's take a step back, and look at all the current knowns:
Oil smoke coming from crankcase breather at a heavy rate.
#4 header said to warm slower than the others.
Some power has been lost.
No mention of any unusual mechanical noises.
The overall cylinder/combustion chamber oil control is by the combination of the valve stem seals and the lower two rings on the piston. (The upper ring provides most of the pressure sealing, the second ring actually providing more oil control that pressure sealing.)
IF your exhaust is not emitting blue smoke, it suggests to me that your overall cylinder/chamber is still reasonably "dry", in other words, oil control has not been lost (at least in the idle mode that you videoed). This suggests that maybe the associated components have not been badly damaged (re the piston that means the ring grooves/rings/walls).
IF your # 4 pipe takes longer to heat, but still gets hot to some degree, it suggests that the peak combustion temperature and / or combustion duration is less. Keep in mind that a big factor in peak combustion temperature is compression ratio, high ratios netting higher peak temperatures.
My guess is that you have at least a holed piston, maybe a previous partial burn that has finally failed.
The only other path for cylinder/chamber gases to get into the crankcase is past a valve stem, and I don't think that is what you are facing.
Pull the # 4 plug and read it for oil and aluminum. It may, or may not, show signs of what has gone wrong, depending on how long you have had those plugs in, and what the failure mode really is.
Do a compression check on # 4 first.
Do a try test first, then a wet test.
If you have a very low number, it matters not what the problem really is, as the engine will have to come apart.
If the wet check yields a major increase in pressure, it typically means that the larger leakage path is past the rings as compared to the valve/valve seat fits.
Then do a compression check on the rest of the cylinders.
See what all the above reveals.
IF your # 4 compression test is very low, and you can feel pressure building up fast and to a level that you can't keep your finger on the vent hose during an idle run test, in my mind there is no question that the crankcase is seeing some serious combustion gas.
One might be tempted to drop the engine and tear into it, but not yet in my mind.
Remove the upper cover as though you were going to check your valve clearances.
Do clearance checks on all valves.
If you have reasonable numbers, especially on the suspect # 4 cylinder, you can reasonably assume that the valve and valve related parts are OK. The chances of having a proper valve clearance in association with a missing valve stem are what I would call low. Some would say this step is a waste of time, but in the past I have used such a check as part of a diagnostics check re bizarre situations (like a CB750 that started and ran, but only two pipes heated up and had plugs being fired by different coils – amazing how a broken camshaft can manifest itself). So why not do it, it’s part of covering all the basics.
Let’s suppose your valve clearances all check out, and you have a duff # 4 compression number.
Get the bike on some kind of stand so the rear wheel is off the ground.
Remove the header.
Drain the oil.
Remove the oil sump.
You should be able to view up and see what is going on.
You might need a mirror.
Use a nice white light for illumination, and LED flashlight would be good.
Use the rear wheel to rotate the engine (use 6th gear) so you can get different views.
You could even probe with a coat hanger re the piston crowns.
When the pistons are on their down stroke, check for piston crown failure.
When the pistons are on their upstroke, check the cylinder walls for scoring.
My guess is that you’ll see something.
I’m really curious as to what is really coming out of your exhaust pipe, as in oil smoke or not.
Good luck !