Let's say that you have to have a certain amount of strength (call it "x") for the tubing on a bicycle frame to be functional. The higher grade the tubing, the thinner walls and lighter weight is possible and still be able to reach that minimum "x" amount of strength. Tange 5 will reach that level of strength at a certain wall thickness, therefore a certain weight. Tange 4, 3, etc. will be able to meet or go beyond that "x" factor with thinner walls because the steel is stronger. Thus the "butted" tubing, aka, double butted tubing (which you don't have) - the tube walls are thinned out in the middle because the steel is strong enough to make the strength requirement without using as much material. The ends are kept thick for brazing purposes, and to handle the stresses at the joined parts.

Bottom line, you've got a decent frame - I've never seen a bad Tange frame yet, be it the cheapest (Tange 5) or the most expensive (Tange 1). Build it up, ride and enjoy.