Deciding to limit my 4k shooting to personal projects and very occassional pro use, I am relishing the many advantages of sticking to 1080p.
The obvious ones do not need to be gone into in great depth. Tons more space, less heat, more battery life, easier processing, are all well discussed, but still real.
The real benefits for me are the creative options that have opened up.
Some cameras shoot 4k well (my G9 for one and the EM1 mk2’s for that matter), some are stretched. Regardless, cameras generally have more options avaialble at lower quality settings*.
The G9 offers VFR or Variable Frame Rate at some settings. I have set three of my custom settings (C3/1-3) to 33% and 20% slow motion and 1250% time lapse. I can access these instantly, giving me direct camera to output control.
The lossless teleconverter is longer, basically twice as long. In 4k it is a handy 1.4x, but in 1080 it becomes a genuinely powerful 2.7x converter. This means that my 75mm f1.8, aready a 150mm equivalent with MFT converting, becomes something close to a 400mm f1.8! I can walk out the door with a single standard zoom and have effectively a 24-200+ f2.8 (FF 5.6)** equivalent. This means buying a prime lens is basically a double buy. For example, if I get the Meike 12mm (24), I also get a 32 (65mm) focal length or the 16mm (32) becomes roughly a 43 (85). This is no small thing, effectively doubling your lens kit and adding versatility at the touch of a button.
Bit rate is deep and all the best things about 4k are accessible in 1080, except crop-able resolution. 1080 ticks along with a ton of quality in reserve. 60 frame 1080 at 100-150 mbs with 10 bit 422 colour depth is better than many cameras 4k output. This is not always guaranteed, but with a Panasonic, there is as much attention paid to 1080 as 4k (Olympus on the other hand does very good 4k, but mediocre 1080).
*This does not mean lower visual quality, just lower potential quantity of that quality.
**2x focal length, but 2 stops deeper depth for the same light gathering.