At the time I'm writing this, I say the Pi 4 runs hot, YMMV This has has been reported many places, including cstanton Christopher's nice benchmark article:
Tom's hardware has some nice tests, checking to see what overclocking it will do.
When I bought my new Pi 4, I also bought the official new Pi 4 case from raspberrypi.org. Why? Because the Ethernet port and the USB ports changed locations, I couldn't use my old Pi cases. I mention this because the new Pi 4 case has no ventilation slots. It's not air tight, so there's some heat escaping, but the heat does really build up inside. The next photo is the Pi 4 in the case, with the top off. The top is the white plastic piece in the upper right corner, you can see the raspberry logo.
I ran some informal tests, using just the indicated temperature shown in the "panel" running in the x-windows interface. The temperatures were noted while viewing the same youtube video under different conditions. The base case was with a stock raspberry pi 4, inside the closed case. Normal clock frequency for the BCM2711 CPU used in the pi 4 is 1500 MHz. The pi quickly went above 81 degrees C, and the other panel widget that shows CPU frequency showed it was then throttled back to 600 MHz. After running at 600 MHz for a while, the clock frequency would jump up briefly to 1500, but would quickly reach 81 C again, so I'd be back to 600 MHz.
I looked through my pile of parts, and came up with some heat sinks, see the next photo. A friend gave me the small one on the left, it was left over from a pi 2 kit that he had.
OK, the massive heat sink on the right was from an Intel server, and clearly won't fit. So I tried the small one first. I was impressed, sticking it onto the CPU dropped the temperature by about 4 degrees C. That kept the CPU running full speed for several minutes at a time. But 75 C is still very hot, touching the little heat sink was almost enough to burn my skin. With the case top put back together, the heat would build up inside the case after several minutes. There wasn't much the little heat sink could do about that. I ran with the top of the case off for a while.
This looks like a Goldilocks problem. ( https://en.wikipedia.org/wiki/Goldilocks_and_the_Three_Bears ). So moved on to the middle heat sink. It was (slightly) too large, it would easily touch the GPIO pins and I didn't want to "brick" my new Pi 4 by shorting something out. It also slightly covered the wifi antenna, and the GPU. I took out my handy hack saw and cut it back a bit.
With it cut down to size, it didn't touch the GPIO and didn't overlap the wifi antenna. I put some electrical tape on GPIO just in case, and some electrical tape on the edges of the heat sink. The heat sink is tall enough that the top of the case won't fit. But this really is an advantage. The Pi 4 case needs some ventilation holes anyway. Some drilling and cutting of the plastic case resulted in a working system. The reassembled Pi 4 in its case is shown below.
I don't have a 3D printer, but of course if you do, a nice printed case fitted for the heat sink would work well for this.
This is a work in progress. I powered up the pi 4 with this configuration, and it idled at 51 degrees C. Before I had added the heat sinks, it idled above 60 C. I did a quick run with you-tube, but didn't have time to let it run more than a few minutes. The photo below shows it running youtube at 55 C.
I still need to run the youtube video all the way through to see how hot it gets, and see if it throttles the CPU speed. But I'm very happy with the results so far.