Inside the parentheses the i is only an integer number, and you get always zero. To access the value of a column inside an expression, use column(i):

plot for [i=1:30] 'file' using 1:(1/column(i)) w lp

The $4 is a shortcut for column(4). To understand this better, have a look at

i = 2
plot 'file' u 1:i, '' u 1:(i)