You say "plainly" because that is what your theology teaches - NOT because the text dictates the meaning. A man can lay down his life for a friend in that persons behalf, for that person, to that person's benefit. I have a friend who lay down his life for his country (killed by a landmine while we were in Bosnia). He did not lay down his life instead of his country.It is not the same at all. In John 15:13 huper plainly means both 'on behalf of' and 'instead of.'
This is very interesting. The word used in John 1:30 according to the T.R. and Majority text is peri, 'concerning,' 'about,' which obviously makes much more sense than, 'on behalf of.' However, that does not change the meaning of John 13:15.
But also, you are wrong about huper only appearing four times in John. Check out, John 10:11; 10:15 (the shepherd dies, the sheep don't); 11:50, 51, 52; 18:14 (one Person dies, the people don't). Greek prepositions have a notoriously large semantic range. Huper does not always mean 'instead of,' but sometimes the context very clearly indicates that it does.
You are pushing the meaning of that passage. The result can be to spare the life of another person but it does not mean instead of the other person. There is a difference between a shepherd dying to protect the sheep, or a soldier dying to protect his country, and the way PSA corrupts Scripture to develop a theory whereby God punishes Jesus instead of punishing us.