It all depends on what your goals are...
Juice, Are you always going to be riding on Big drag radials?
Like Esptein said,
If you're going to run a 28" tire, you'll need a 4.636 just to get back to the stock equivalent (25" tire, 4.083).
But you stated you were running in the 26" size and you want to rev to 10 Grand.. the 4.9 might be your ticket...
I would go with a 4.36. It's a happy medium between the stock 4.08 and the 4.9. I'm about a 25 & 1/2 tire with some meat, power estimate around high 400's. the 4.36 should suit me well, at the expense of some added traction loss
Car should FEEL faster though...
You can find the pumpkins in the skyline..... what, gts diffs and the early Ca powered 180's right? Best way to find out you have one is take off the diff cover and look at the ring gear. It should have a 48:11 ratio stamped in. calculate that and it comes out to 4.3636_
Goodluck man