[a_opt, Fval, exitFlag] = particleswarm(fun,nvars,lb,ub);
The first output from particleswarm is a row vector with length equal to the number of variables -- so 1 x 4 in your case. The second output is the scalar function result.
You use conjugate transpose on the 1 x 4, which will give you a 4 x 1. You use it with  with the scalar Fval, so you are trying to horizontally concatenate a vector with 4 rows and a vector with 1 row. That is not a permitted operation.
You need to either not transpose a_opt and lb, or else you need to use ; instead of , in the  so that you construct a column.