Please read the documentation of assignin. There you find the explanation, that the 1st input is either 'base' or 'caller'. But you use a function call to the script, which starts this script for an evaluation. The appearing error message should be clear.
Better define an output argument:
function [A, x, B] = fd4(L, N)
...
Then call this function from your script like this:
[A, x, B] = fd4(L, N);
Then B is assigned as wanted without brute tricks as assignin, which impedes debugging due to magic remote creation of variables. A reader of your script cannot guess, where B comes from, when reading the code.
