an EF-M43 adapter. There is also the possibility of special adapters, so-called focal reducers from the companies Metabones ("Speedbooster") and Viltrox. I haven't used any adapters myself yet, I'm only working with M43 lenses for the time being. I suggest to inform yourself in the BMD Forum about the advantages and disadvantages of focal reducers. In a nutshell: Advantage is more light, disadvantage can be loss of sharpness at full open aperture, as well as moiré effects. But it always depends on which lens is used and which focal reducer. Metabones recently even developed a speed booster especially for the Pocket 4k. I would use this if I had to.

Due to the somewhat larger sensor compared to the M43 sensor, this leads to a "bent" image with wide-angle lenses, e.g. 12-35mm or 17mm. This is very annoying because you have to correct it in post-processing and lose focal length. I never noticed this with Speedbooster.