As someone who does a lot of video editing, I would NEVER expect a Sound Designer, Mixer or Engineer to be working with a full resolution version of the video. Never, especially 4K. You need the processing power for your VSTs and your DAW, not video playback. 4K video will also be thrashing your hard drive and seriously affect the ability of your computer to keep up with data IO.
Using ProRes rather than H.264 is a good as it is less processor intensive but you should be getting a ProRes LT file at 1080p or even 720p. Make sure you match frame rates and both be working in either DF or NDF in your settings and his project. Just be the same and consistent. Frame size, however, has no bearing on your ability to collaborate. When you import the video into Nuendo it should pickup the frame rate and nature of the timecode of the video you are working on.
One of the Steinberg reps or developers if they chime in here, can tell you have efficient the Nuendo video engine is at working with H.264 files. It may in fact be fine. Also the length of your video may play into things. If it is a commercial or a two hour video that could easily change the way you approach encoding and sizing your video. Still a 4K preview video in Nuendo regardless is not necessary or considered standard workflow for sound design or mixing.