. . . . video at 4k res and I changed the project to that when I inserted . . . . editing is slow and jerky so want to change the project back to HD. . . . .
Changing the project resolution will not help with this - your source video is still used as 4K which requires more processor power hence the slow editing and jerkiness.
Do you have Create proxy files turned on in the projects settings - if not turn them on - do note that the proxy files will take up additional disc space, and they will take time to create. However once created they are easier to work with and playback should be smoother