How much eye relief do I need on a scope for a 336 30-30? The one I am looking at comes in 4 or 6 inches. Thanks.

The .30-30 is not a particularly hard kicker, so 4" should be sufficient. (BTW, most scopes have a "range" of eye relief. Are you sure that's not 4 to 6", rather than 4 or 6?).
The real answer will depend on exactly how you mount the scope and how you use it. Some folks are "stock crawlers" -- in other words, they tend to put their faces far forward on the stock. If a scope has sufficient eye relief this may not matter 90% of the time. But from time to time in the hunting field you'll encounter situations where your shooting position may have to be a bit contorted and clumsy -- and in those situations what was "enough" relief for the last 100 rounds fired may be enough to cause you to end up needing stitches in an eyebrow. Be sure to position your scope so that you have sufficient relief to move your eye a bit forward or back from your normal, comfortable position and still have a full field of view and sufficient clearance. This should be enough to save you from acquiring an intersting scar.

Just curious. I have seen scopes before that just fell into place when you shouldered the rifle and others that you had too move around to get into position and I thought that eye relief mught have something to do with that. Most seem to have around 4" but one that I looked at was available in 6" as well. The 6" would work fine wouldn't it? Thanks for the help.

If you mount it in the standard way, on top of the receiver, 6" may be too much. When I mount a scope, I first put the scope in place without tightening the rings all the way down on it. Then, I shoulder the rifle naturally and look through the scope, sliding it forward and backwards in the rings until I have it positioned where I have a full field of view.I then tighten things down and check again to be sure the scope is positioned so that my I get a full field of view every time I shoulder the rifle, with no adjustment needed. Sometimes I may have to loosen it up again and slide it a bit fore or aft, until I get it just right. Most mounts will allow a scope to be moved at least and inch forward or back from dead center, and this adjustment is the reason why.

Most scopes with an eye relief of 6" or more are really designed to be mounted forward of the receiver in "Scout" style.

The XS Lever Rail will allow you to mount a scope of any eye relief you wish.

http://www.xssights.com/store/scope.html

Conventional rifle scopes usually have around 3.5 to 4.5 inches of eye relief. On the Marlin lever rifles regular scopes require the use of a hammer extension due to scope overhang.

Scopes with 6 inches of eye relief mount with the end of the scope flush with the back of the receiver. No hammer extension is needed. These are not scout scopes.

Scopes with 9 to 12 inches of eye relief are actual Scout Scopes. The back of a scout scope should be about even with the front of the rifle receiver. No hammer extension is needed.