I'd bet you don't even NEED grain -- it's built right into the film frames.
But here's the big issue: how are the home movies converted? What's the horizontal & vertical resolution of the video files you work with?
If you're ripping a DVD to get them, going to 4K isn't going to make them look any better. Just bigger.
If you movies were not transferred with a telecine setup and are standard def I wouldn't try upscaling them. You'll end up with a mess. If you used a real telecine service to encode the home movies to BluRay or HD digital files with h.264 or similar MPEG encoding I'd probably just leave them alone. Only if you had your original films encoded with a real telecine and have what would be considered a production codec copy would I try to use AE's improved upscale or something like Magic Bullet's Instant HD to uprez. I can't see any situation where I would take previously encoded 8mm movies and try and push them all the way to 4K for several reasons. The first is it's too far to go to expect any kind of quality increase. The best you can hope for would be a zero gain in resolution or clarity. The second, You've got to have some fairly fancy hardware to successfully playback 4K files to a 4K monitor. Third, The final viewed image of a HD source played back on a decent 4K monitor will never look any better than HD broadcast TV or BluRay playback through the monitors automatic resizing capabilities. There's only one way to make 8mm film look better than a true telecine conversion to HD and that's a true telecine conversion to 4K. There's just not enough information in the negative to make it worth the effort.
And what Dave said. You don't need any more grain.