A friend of mine over here has a 01 TJ (European Spec). We are going to be putting on a Tera 4" Long Arm but I imagine that the trac bar will give us some problems. It uses a different bracket than US spec one do and also looks to be bent in different places and is shorter than my 98 TJ. Having a custom trac bar made up is no problem but how does one determine what length over the stock trac bar to make it to compensate for the 4" of lift.

I'd rather do a custom trac bar than extend the bracketry.

Is it as simple as using a^2 + b^2 = c^2 where a is the distance inboard of the frame for the axle bracket and b is the height off the axle for the frame bracket (to be increased by 4" over stock figure to compensate for lift) and c is the trac bar?

Sure, you could use old pythagoras to figure it out. Make sure the "normal" weight is on the vehicle and the axle is centered when you do your measurement. The beauty of the adjustable trackbar is you don't have to be exact since you have adjustment.

http://www.nagca.com/adjustablereartrackbar.htm