Out of my 7 1/2" SRH 21.0 gr of H-110 sparked off with a CCI-350 will push a 390-410 gr cast +/- 1200 fps. I'd expect a 4 5/8" barrel to loose 100 fps, and I've heard the recent generation of Ruger 480's are ~100 fps slower than the early SRH's though I have no idea why.

If you're looking for a bit more out of your 480 you can get a custom mold cut that will allow you to seat the bullet out to take advantage of the length of the cylinder.

[Linked Image]

Should be good for an extra 100-150 fps, if you can handle the recoil. When pushing 400 gr, you notice an increase in recoil with a mere 50 fps. When you jump 150 fps, you really notice the recoil.