I know the most im gonna get out of the 7.62x39 is 400 yards simply because at 500 it typically drops about 85 inches. How accurate is my sks going to be at 400 yards with a 4x scope? its a chinese 56.

It's going to depend on the rifle, ammo, shooter, and scope.  My guess would be if everything is working correctly about 4MOA, which at 400 yards would be a 16" group.  You might be half that if everything is really good.  You might be 2 or more times that if the gun & ammo combination doesn't work well.  The only way to find out is to shoot it.  

at 50yds im getting about 2 moa which, all things considered, isnt bad. I'm jsut shooting wolf military classic HP. I must say though that the NC star scope i got for 16 bucks is rather poor. it is holding its zero well but the crosshairs are so thick its rediculous.

it is holding its zero well but the crosshairs are so thick its ridiculous.

The Leapers 3 x 9 x 40 compact scopes are not  any better cross hairs are to thick on those also,I just put a 3 x 9 x 32 Bushnell I had on my YUGO its like nigh and day.  I average 50 Yd. 5 shot one ragged hole groups with this combo and 1" or less out at 100 Yds with my reloads.  The affective range of the 7.62 x 39mm is more like 300 Yds than 400,if your planning on hunting with it as I do try to keep your shots within 150 Yds or less.

I try to put myself in a position  to make shots that are well under 100 yds like I would if I were hunting with a 30-30.  For target shooting the only limits are your abilities and your equipment. As far as using a 4x scope at 400 Yds you want see your hits on paper through it much past  150Yds.