You need to concatenate( in the else as you currently have it):
str = [str sprintf(' + X(%04d) ',i)];
Why do you have the counter variable though? You already have a loop variable i that can do this. Though personally I would initialise the string with the unique first case outside of the loop. There's no need to test whether you are on the 1st iteration every single iteration of the loop when you know for sure it will only be true the first time. The performance cost is minimal here, but still, you don't want to do things in a loop that don't need to be done in a loop, as general good practice.
